At best, feeling cold makes kids grumpy, at worst, ill, so slipping thermals under their clothes is an simple way to make winter a lot easier on everyone.

Traditionally, thermals have been designed to sit close to the skin and use the body’s natural body heat to stay warm.

Nowadays, there’s no shortage of choice with everything from natural 100 per cent merino wool to hi-tech polyesters being used in a range of designs.

When it comes to finding the perfect set for your kids, you need to consider what they will be doing in them: the thermals you choose for a week away skiing should not be the same as those you opt for to run around the garden in.

Our little testers spent hours trying out a range of kit during a variety of activities for this roundup. Here's our top pick.

You can trust our independent reviews. We may earn commission from some of the retailers, but we never allow this to influence selections, which are formed from real-world testing and expert advice. This revenue helps to fund journalism across The Independent.

Kozi Kids vasa striped baselayer set: £29, Kozi Kids

Bright, lively and stripy, this base layer set is designed to make wearing thermals a fun, enjoyable experience for kids. Top and bottom are 100 per cent polyester but the texture is quite unlike the other man-made base layers we tested: the surface is almost shiny so they feel silky and incredibly soft. After being put through their paces during a long evening at the Christmas lights, they got a big thumbs up in the warmth department. Being thin also makes them a good option if you have a lot of layers to contend with, plus the slightly raised neck adds extra draft-proofing against the cold. 

Buy now

Superfine merino base layer set: £59, Superlove Merino

The presentation box and tissue paper that Superlove Merino thermals come wrapped up in are the first clue that these are a little bit special. They’re cut to "hug" the skin, so the 100 per cent superfine merino wool can naturally regulate body temperature, but they’re still easy to get on and off irritable toddlers come bath time.

If you’re looking for ethical base layers, these tick the box. Made in the UK, the brands products come courtesy of happy, free-ranging sheep and there are no chemical nasties. At £59, it’s an investment but a worthwhile one: they’re in a league of their own when it comes to softness and they made a two hour wet and windy walk a lot less painful.

Buy now

Columbia youth baselayer top: £27, Columbia

Despite looking like a disco ball, the sparkling silver lining on this base layer top is actually there to be reflective, directing heat back towards the body. It works well and feels like a second skin; thin but unobtrusive. The features don't end here, this 100 per cent polyester top also has moisture-wicking technology and anti-microbial treatment to prevent odours. Certainly, after a few hours of running around, there was nothing to sniff at. 

Buy now

Joha thermal leggings: £26, Childrensalon

Trust the Danes to know what to wear when it’s cold. Joha’s long johns are surprisingly thin for thermals and are made from a combination of wool (for warmth) and silk (for softness). There’s an old-school lace design and trim which gives them an on-trend hygge vibe. Our little tester declared them the most comfortable and refused to take them off even when back inside. Thanks goodness they wash well at 40 degrees (and dry quickly) because they were demanded the next day too.

Buy now

Polarn O. Pyret thermal merino kids top: £24.50, Polarn O. Pyret

Stylish isn’t the first word that pops into one’s head when thermals are mentioned, but Polarn O. Pyret has given its heat-regulating top a fashionable nod so it can be worn on its own as well under clothes. Good looks aside, as a piece of thermal clothing it fairs well too. Made from 100 per cent merino wool, it is warm and soft and the dipped back hem, which can be easily tucked into trousers, is a clever extra. It needs to be treated with respect though, and has to be washed on a 30 degrees wool cycle to keep it looking smart.    

Buy now

Mountain Warehouse merino round neck base layer top and pants: £14.99 each, Mountain Warehouse

While this merino-blend set doesn’t win any awards in the design department, if you’re after good-value, decent, everyday thermals, this set is a sensible choice. It ticks the boxes for warmth, comfort and softness and washes well at 30 degrees. A slight criticism would be that they were a little baggy and too big around the gusset, but the testers didn’t seem to care, even when wearing them under a pair of jeans.

Buy top now
Buy pants now

Kuling neck warmer: £9.60, Alex and Alexa

Don’t be put off by its fineness and unusual design; this 100 per cent merino wool neck warmer will 100 per cent keep them warm. It slips on over the head and is sleeveless, allowing ski-loving juniors the chance to stay cosy while having the freedom to move without restriction. That said, even if you’re not hitting the slopes en famille this winter, this is a good choice for kids who hate wearing lots of layers.

Buy now

Tresspass bubbles microfleece base layer: £14.99, Trespass

These fleecy base layers are made from polyester and kept everyone warm and cosy, even when the sun went down and the temperature plummeted. The bright colours were a hit with the kids, while the quick-drying properties (no need to tumble dry, just hang up) met with adult approval. They do leave the skin a little clammy so it’s worth leaving the half zip open and allowing it to breathe.

Buy now

Icebreaker kids’ oasis crewe top: £24.50, Icebreaker

While not the most exciting of thermals, this 100 per cent merino top is designed to be worn in seriously cold climates. Despite feeling thin and almost insubstantial to touch, this is a thermal that should not be underestimated; it was tested on a freezing day in the British countryside and passed with flying colours. It also boasts odour-resistant properties and apparently can be worn day-after-day on a hiking trip without getting pongy. But we drew the line with testing this.

Buy now

The verdict: Kids' thermals

Kozi Kids’ thermals are the ideal mix of high value, hi-tech and hi-jinx and make putting the kids in an extra layer a much less odious task. But, if you’re after something really special, you can’t beat Superlove’s merino set, aka luxury in thermal form.

IndyBest product reviews are unbiased, independent advice you can trust. On some occasions, we earn revenue if you click the links and buy the products, but we never allow this to bias our coverage. The reviews are compiled through a mix of expert opinion and real-world testing.