>>56501336That is a very womanly figure for a 10 year old. Which is perfectly fine.
I think they do that because the characters being 10 is just so it stays under the umbrella of "kid franchise" but they know a very large portion of the fanbase aren't kids and haven't been kids for decades so they design a more mature version of the character to appeal to this part of the fans.
Pokemon has lots of adult and lots of children fans so they are trying to appeal to both worlds by designing a mature, visually pleasant female that is very a much a woman in some shots, but then just saying "hey kids, she's 10, she's just like a you, a kid going on a big adventure in the pokemon world!".