My DD are swimmers/ divers. If you buy a suit in competitive sizing you will have more luck. They are different than the typical xs, s, m etc or 0,2,4. If you are a xs, get a 26. Swimsuit outlet is a great place to get a variety of suits at different prices. This particular suit has been a favorite for my diver and my swimmers as a practice suit and it’s relatively inexpensive. The same brand has more simple color choices as well:
https://m.swimoutlet.com/p/sporti-molecule-thin-strap-one-piece-swimsuit-8132050/?color=9755
Good luck. A 24 would be like a girls 10, my oldest DD gets a 26 for competition but it’s super tight on her. For practice she’s a 28. She typically wears smalls and 2s but has a really athletic build and a long torso. Her taller leaner younger sister also wears the same sizes is usually an XS and 0-2 in women’s sizes. The 26 is snug on her. She also wears a 28 for practice (she’s 5’8” and 115lbs). The youngest who is 12, about 92lbs and 5 feet can wear the women’s 26 for competition and sometimes steals one of her sisters’ 28s for practice. So, there’s a big range but I think you’ll find they stay on and fit well. A lot depends on how snug you like your suit. They do stretch. Good luck! Hopefully this helped a bit!