What roll do your sons have in selecting their clothing?
They are old enough to have a say in what they want to wear and what they feel comfortable wearing. They are at an age where they may be aware of whether their clothes are like those of other children or not.
If your child wants to wear leggings and a t-shirt, it is harmless enough. On the other hand, if you're dressing them in an odd manner to suit your own tastes, you may be putting them in an awkward position. If the three girlfriends are commenting about it, nursery workers may be, too. When I was little, my mother cut my hair short because it was easier than dealing with wild curls and dressed me in my brother's hand-me-down clothes because money was tight and those clothes were still "perfectly good". I remember a couple of nursery school workers having a conversation over my head about how my mother should be ashamed to send me to school looking like a little boy. One called it "disgusting". At 3 or 4, I was content with my practical playclothes and easy hair, but their comments confused and embarrassed me.
None of my own kiddos wanted to wear jeans at all at that age, much less skinny jeans. They very much preferred pants and shorts with elastic waistbands because they were easier to manage and more comfortable when sliding, crawling around on the floor with toy cars, and all those other things little kids do in their busy day. Long underwear, such as WinterSilks, is probably a better choice than tights on a very cold day.
Other people shouldn't tell you how to dress your children, but every day play clothing choices should be made based on what makes your kids happy and comfortable. They aren't babies to be dressed in mom's choice of cute outfits anymore, so it is time to start giving them a chance to make some of the choices about what they want to wear and what they do not.