I get this question a lot from new dancers and parents of little ones just starting ballet. Some people believe it's immodest to go commando under dancewear, and others think pants are necessary for proper hygiene. The plain and simple answer to this question is no, it's not appropriate to wear underwear beneath a leo and tights. In this post, I'll unpack why.
Note: In this post, I'm discussing mostly young girls and children's dancewear and classes. For more on dancewear and underwear for women, see my post on dancewear for periods here and my post on leotards and bras here.
Underwear shouldn't be worn under leotards and tights for a few different reasons. The first is that it's simply not needed. Both leotards and dance tights have breathable hygienic linings and gussets. In leotards, you may notice the crotch area has an extra piece of white, tan, or black fabric sewn in. In tights, you will see an oval-shaped piece at the crotch that has a slightly different texture than the rest of the fabric. If a leo or pair of tights do not have a hygienic lining or gusset, it's not a real piece of dancewear and is not appropriate for class.
Second, wearing underwear beneath leotards and especially beneath tights is uncomfortable. Kids often wear pants that are slightly baggy, and they get creased and can feel lumpy when sung dance tights are pulled on over top. When I teach children's ballet, I notice girls who wear underwear are more likely to get distracted by adjusting their clothing than girls who wear just tights and leos.
Third, it's not a good look to have underwear poking out under (or neon colors visible through) a leotard. When dancers are in higher level classes, they must obey strict dress codes where it is prohibited to wear underwear beneath dancewear. It's best for young students to get used to this early on. Additionally, visible underwear draws the eye to a student's waist and hip area. It's actually much more modest for girls to wear just their dancewear so the teachers, students, and others can focus on dancers' smiling faces.