It's insanely warm. My 3 year old goes to a forest school, we live in Kentucky so it's not insanely cold but it gets down to 10(F) and his teachers actually asked me to stop wearing the t-shirt at all because he cried, it was hot and sweaty at 15 degrees and 5 cm of snow π . So far it's holding up great. Runs a little small, or maybe very true to size, other than that it should work with the base layer. It fits him well now, but I factored in room for growth and not a lot of extra space.
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