Cot and Children’s Mattresses: Fit, Firmness and When to Size Up
Affiliate disclosure: Sleeping Express may earn a commission when you buy through links on this page, at no extra cost to you. We have not tested every mattress mentioned; guidance is based on published specifications and UK standards. This article is general information, not medical or safety advice — for infant sleep safety, follow current guidance from The Lullaby Trust and your health visitor.
Buying a mattress for a child covers a wider range than most people expect. A cot mattress for a newborn, a toddler bed, and a single mattress for a nine-year-old are three quite different products with different priorities. What links them is that fit and firmness matter more here than they do for adults, and the margin for getting it wrong is smaller.
This guide runs through cot mattresses, the move to a bed, and what to look for as children grow.
Cot mattresses: fit and firmness come first
For babies, the two things that matter most are that the mattress is firm and flat, and that it fits the cot properly. Official UK safer-sleep guidance is consistent on both points. Soft or sagging surfaces are not recommended for infant sleep, and gaps around the edges are a hazard.
The usual rule of thumb is that you should not be able to fit more than about two fingers between the mattress and the cot frame. Measure the internal dimensions of your cot before ordering rather than assuming a standard size — cot and cot bed sizes vary between manufacturers.
You will see foam, fibre and sprung cot mattresses. All can be suitable if firm and correctly sized. Foam is usually cheapest, sprung tends to last longer through a second child, and fibre sits in between. What matters more than the type is that it holds its shape and does not develop a dip.
Standards to look for
UK mattresses for children’s furniture are commonly tested to BS EN 16890, and fire performance to BS 7177. A reputable retailer will state which standards a product meets. If a listing is vague about this, ask.
Second-hand cot mattresses
Guidance generally advises using a new mattress for each baby where you can. If you do reuse one, it should be firm, flat, clean, completely dry, free of cracks or tears, and it must fit the cot you are using it in. A waterproof cover helps considerably with keeping a mattress usable and hygienic.
Moving from cot to bed
Most children move to a bed somewhere between 18 months and three and a half years, usually prompted by climbing out of the cot rather than by age. A cot bed extends this by converting, which is why cot bed mattresses are longer than standard cot mattresses.
For a first proper bed, a shorty or small single is often a better fit than a full single, both for the room and for the child. UK shorty mattresses are typically 75cm x 175cm, and small single is 75cm x 190cm.
Mattresses for school-age children
From around five or six, children can use a standard single mattress, and the choice starts to resemble an adult purchase — with two differences.
First, children weigh less, so a mattress that feels medium to an adult will feel firmer to them. A medium or medium-firm rating is usually a sensible target rather than anything described as firm or orthopaedic.
Second, children’s mattresses take more abuse. They get jumped on, slept on diagonally, and occasionally rained on from above. Durability and a washable, waterproof protector matter more than a premium comfort layer.
Sizes and what they suit
| Size | Dimensions | Typical stage |
|---|---|---|
| Cot | Varies — measure your cot | Newborn to around 18 months |
| Cot bed | Longer than a cot mattress; varies by frame | Converts to a toddler bed |
| Shorty | 75cm x 175cm (2ft6 x 5ft9) | First bed, small rooms, cabin beds |
| Small single | 75cm x 190cm (2ft6 x 6ft3) | Older toddler through primary school |
| Single | 90cm x 190cm (3ft x 6ft3) | School age into the teens |
Where to look
The Sleep People cot mattress range and their Starlight Babies line cover the cot and cot bed end. For older children, Starlight Beds children’s mattresses and shorty small single mattresses cover first beds and small rooms.
For cabin beds, mid-sleepers and any frame with a non-standard base, kids’ foam mattresses can be made to size, which avoids forcing a standard mattress into a frame it does not suit. All of these brands are part of the same West Yorkshire group rather than independent competitors.
A further option approved since publication: 5 Little Monkeys make mattresses specifically for children rather than adapting adult builds, which is worth a look alongside the ranges above. As with any cot or child bed purchase, check the stated size against your frame and confirm which standards the mattress meets.
A short checklist
- Measure the cot or bed base internally — do not assume a standard size.
- For cots, aim for a snug fit with no meaningful gap at the edges.
- Choose firm and flat for babies; medium or medium-firm for older children.
- Add a waterproof protector from day one.
- Check which standards the mattress meets, and ask if it is not stated.
- Replace rather than reuse where you reasonably can, and never use one with a dip, tear or damp.
Frequently asked questions
How firm should a baby’s mattress be?
Firm and flat. Safer-sleep guidance in the UK is consistent that soft or sagging surfaces are not suitable for infant sleep. If you can press it and leave a lasting impression, it is too soft.
Can I use a cot mattress in a cot bed?
Usually not, because cot beds are longer. Measure the frame and buy the size it takes rather than adapting.
When should a child’s mattress be replaced?
There is no fixed interval, but replace sooner if it dips, has a tear, or has been persistently damp. Children’s mattresses tend to wear faster than adult ones for their price bracket.
Is a shorty mattress worth it or should I go straight to a single?
If the room is small, a shorty genuinely helps and children rarely outgrow one before school age. If space allows, a small single or single lasts considerably longer and avoids buying twice.
The short version
For babies, fit and firmness are the whole decision — measure the cot and buy new where you can. For toddlers, match the mattress to the frame rather than the age. For school-age children, treat it much like an adult purchase but aim a step softer and spend the difference on a decent waterproof protector.
