When should kids wear undershirts
It really depends on your preference and budget.
My mom always made us wear undershirts in the winter time. It's always good to layer in the winter time since it keeps you warmer. My husband wears them but I've only had our children wear them in the winter or as light pjs in the summer time.
My son refuses to wear one. He won't even layer clothes and getting him to wear a jacket that is comfortable for him can be.. trying.
Honestly, go by the comfort of your kiddo. If they are comfortable with it then sure, have them wear one. If not, it's not worth the fight in my opinion.