I live in Los Angeles where it is warm compared to most other places and I still put a t-shirt under my ds' shirts. We live by the beach so it is pretty crisp in the mornings. Many days his teacher takes his top shirt off by the end of the day and he is running around in his white t-shirt, but at least I know he isn't cold in the morning. I get them in 3 packs at Target.
Re: Do you put a tshirt under your toddlers shirt??
Yes, always. My boys wear the "wife beater" tshirt under their shirts.
Oh, and yes....at WalMart
Ditto - adds warmth without too much bulk. On the baby I still use onesies.
In the winter, we do. Right now, we have some from Children's Place and Carter's.