Should I Move My 5-Year-Old to a Booster Seat? A Practical Guide for UK Parents

By the time your child turns five, you’ve probably spent years buckling them into a forward-facing car seat with a five-point harness. It’s snug, it’s secure, and honestly, it’s starting to feel like a tight squeeze. You’re not alone if you’re wondering: should I move my 5-year-old to a booster seat? The answer isn’t just about size-it’s about safety, law, and real-life comfort. Let’s break it down so you can make the right call for your child.

UK Law Says: Stay in a Harness Until You’re 15kg or 125cm

In the UK, the law is clear: children must use a child car seat until they’re 12 years old or 135cm tall-whichever comes first. But the rule for when to switch from a harnessed seat to a booster is more specific. You must keep your child in a car seat with a five-point harness until they weigh at least 15kg (about 33 pounds) OR reach a height of 125cm (about 4 feet 1 inch). Most five-year-olds haven’t hit either mark yet.

Here’s the reality: the average five-year-old in the UK weighs between 16kg and 20kg and stands around 110cm tall. That means many kids this age are legally allowed to move to a booster. But legality doesn’t always mean safety. Just because you can switch doesn’t mean you should.

Why a Harness Is Safer-Even for Big Kids

Five-point harnesses don’t just hold kids in place-they spread crash forces across the strongest parts of their body: shoulders, hips, and pelvis. Booster seats, on the other hand, rely on the adult seatbelt to do that job. But seatbelts are designed for adults. For a small child, the lap belt can ride up over the soft belly instead of sitting low on the hips. In a crash, that increases the risk of serious internal injuries.

Studies from the UK’s Department for Transport show that children in harnessed seats are up to 50% less likely to suffer serious injury in a crash than those in boosters, even when they meet the minimum weight and height rules. That’s not a small difference. It’s the difference between a trip to A&E and months of recovery.

Think about it this way: your five-year-old’s bones are still growing. Their pelvis is narrow. Their spine is flexible. A harness keeps them properly positioned. A booster? It just lifts them up so the belt *might* fit.

Signs You’re Ready for a Booster (And Signs You’re Not)

There are real, observable signs that tell you when your child is ready for a booster-not just numbers on a scale.

• You’re ready if: Your child weighs over 18kg, is taller than 125cm, and can sit still with the seatbelt fitting correctly-shoulder belt across the collarbone, not the neck; lap belt low on the hips, not the stomach.
• You’re not ready if: Your child slouches, fidgets, or pulls the belt behind their back. If they still need a headrest to sleep in the car. If they’re still in a seat that feels tight around the shoulders or thighs.

One parent I spoke to in Bristol switched her five-year-old to a booster because he complained about the harness being “too hot.” Two weeks later, he was slouching so much the seatbelt was across his neck. She had to switch back. That’s not rare. Kids this age aren’t always good judges of what’s safe.

Booster Seats: High-Back vs. Backless

If you do decide to move to a booster, the type matters. High-back boosters are the better choice for five-year-olds. Why? Because they guide the seatbelt properly and provide head and neck support-especially important if your child falls asleep in the car. Backless boosters are cheaper and lighter, but they only raise your child up. They don’t help with belt positioning or head support.

UK safety standards require high-back boosters to have side impact protection. That’s not a luxury-it’s a necessity. In a side collision, your child’s head is at risk. A high-back booster acts like a shield.

Look for boosters with adjustable headrests and shoulder belt guides. Brands like Britax, Maxi-Cosi, and Cybex all make models that meet ECE R44/04 or the newer i-Size (R129) standards. Always check the label.

What About Those ‘All-in-One’ Seats?

Many parents buy seats that grow with their child: rear-facing → forward-facing with harness → booster. These are called combination seats or all-in-ones. They’re expensive, but they can save you money long-term.

Here’s the catch: many of these seats have a higher weight limit for the harness-up to 25kg. That means your child could stay in the harness until they’re seven or eight. That’s not just safe-it’s ideal. If you’ve got one of these, don’t rush to switch. Let your child stay in the harness as long as they fit.

One mum in Bristol kept her son in his all-in-one seat until he was eight. He was 132cm tall and still used the harness. She said, “I didn’t care what other parents thought. I cared that he was safe.”

How to Test If a Booster Fits Right

Before you make the switch, do this simple test:

1. Have your child sit in the booster with the seatbelt on.
2. Check the shoulder belt: it should lie flat across the collarbone, not the neck or arm.
3. Check the lap belt: it should rest low on the hips, touching the tops of the thighs-not the stomach.
4. Have them sit upright. Can they stay like that for 10 minutes without slouching or fiddling?
5. Can they reach the seatbelt buckle without help?

If you answer “no” to any of these, stick with the harness. No rush.

What If My Child Says They’re ‘Too Big’ for the Car Seat?

That’s normal. Five-year-olds are starting to notice what other kids are using. They see older siblings in boosters. They want to feel grown-up.

Instead of giving in, reframe it. Say: “You’re not too big-you’re still growing. This seat is keeping you safe while you grow. When you’re tall enough, you’ll get a booster that feels just right.”

Let them pick the colour or design of their next booster. Give them a sense of control. But don’t let them choose the safety level.

When in Doubt, Wait

The biggest mistake parents make is switching too early because it’s convenient. Or because their child is “big for their age.” But safety isn’t about convenience. It’s about protection.

If your five-year-old is under 15kg or under 125cm, stay in the harness. Even if they’re at the top of the weight limit, if they’re not yet 125cm, keep them in it. Height matters more than weight when it comes to belt positioning.

Most kids don’t hit 125cm until they’re six or seven. That’s okay. There’s no prize for switching early. There’s only risk.

Final Thought: Safety Isn’t a Milestone-It’s a Habit

Car seat safety isn’t about checking boxes. It’s about building habits that last. Your child will remember how safe they felt. They’ll remember the consistency. And when they’re older, they’ll pass that on.

So if you’re unsure? Wait. Test. Watch. Ask your paediatrician or a certified child car seat technician. There are free fitting checks available across the UK through the Royal Society for the Prevention of Accidents (RoSPA). You can book one online in minutes.

There’s no rush. Your child’s safety doesn’t have an expiry date. It has a timeline-and it’s longer than you think.