I think the OP meant girls size 16/18, which would be around a small or medium in juniors or misses. I am actually looking for the same type of thing, but for clothes. I would love to have "girlier" clothes, as in little girl designs.
The issue becomes that girls sizes are meant for prepubescent shapes and juniors and misses sizes are meant for more full-figured ladies. So even if you are small enough to fit into them, they may be too tight in the chest/hip or if they fit in the chest, too loose around the waist. I think for swimsuits, it would be more of an issue with them being too short from chest to hip and also having your breasts popping out. It might also be cut too slim in the front and back for appropriate coverage if you were planning to go to a public pool or anything like that.
Definitely let me know what you find out!