My mom has always taken my kids (she brings them to Mass every Sunday), but it's casual here. My kids have always been advised to get on line with the priest with the shortest one. First Communion, however, is a whole different ballgame - the girls wear dresses that cost hundreds of dollars, and get their hair professionally done, and the boys wear suits. For Confirmation, the girls wear nice dresses, and the boys wear nice pants and button down shirts, or suits.
My kids wear jeans, shorts, sports uniforms (if there is a game) to Mass, but it's an informal children's Mass.