I know thermal underwear needs to be snug and I specifically ordered a size for my 2 year old girl but it was still too small. I ordered her a size 4 and she usually wears a size 3. I was expecting them to still be tight but the shirt is short making her look tiny when she raises her arms a little and her pants are a little longer than capris. But they seem to be well made. Don't forget to order a few sizes!