Keeping kids warm when they're outside is super important. They're not going to tell you when they're cold, certainly not in a timeframe you can use to deal with it. That means you need to get invest in good quality layers everywhere for them.
Kids also grow so fast, so getting really nice layers all around all the time gets expensive quick. Your outdoor kids still need all the other clothes that kids need so it's not like because you purchase great base layers you can skimp on the rest of their wardrobe.
Finally, so few outdoor stores really stock a good selection in kids gear. Even our very outdoors oriented friends cut back so much when they had kids and regularly just make do with some cotton clothing on a day hike. While you can get away with less than ideal layers on day hikes, once you start heading out overnight that totally changes. Since there is a smaller market for kids gear, it's made less and stocked less. You're possible left at the mercy of whatever polyester layer the local kids clothing store has as the weather turns cold.
Enter the MEC Base Layers
This year for our almost 3 year old we purchased two new base layer items. For her top we got the MEC Midweight Underwear Turtleneck and for her bottoms we purchased the MEC Midweight Underwear Long Johns. Combined these two pieces cost just over $50 and they've been doing a great job keeping our kid warm as we adventure.
We went with the turtleneck version of the top because both Cynthia and I love the extra warmth gained with the collar. This did create a fit issue though. The turtleneck is so tight that it becomes a wrestling match to force it over her head. Luckily our girl thinks it's hilarious because if she resisted the top wouldn't ever get on. Once it's on, this is a great fitting top though. Great length on the sleeve provides full coverage without ending up in everything she eats on the trail. Some room in the torso but tight enough that it's not letting cool air inside all the time.
One thing you notice quickly in poor quality base layers is that they start to smell really quickly. So far with a few trips under our belts these midweight layers from MEC don't seem to get smelly any faster than the nicest pieces that the adults in our house own.
Yup these are nice pieces of gear for kids. The one change I'd like to see is making the head hole slightly larger. If you tapered the neck so it was the same size and the base was just a bit larger then the shirt would be much easier to get on.