Winnie Cooper in The Wonder Years, played by Danica McKellar, who seems to have turned out pretty well as an adult. The first season she was just amazing, but even in the later seasons she was still attractive.
Samantha in Who's the Boss?, played by Alyssa Milano, who took a wrong turn somewhere. Again, especially the first season.
Missy Cooper in Young Sheldon, played by Raegan Revord. I hope she'll turn out well. There seems to be something positive going on for girl roles with the Cooper family name, so hopefully it will extend to real life results too.
Penny, in the original Lost in Space, played by Angela Cartwright. She was a little too old in this role but still gorgeous, but it's the role I first knew her from. For an even better role see her appearance as Jane in Babes in Toyland (1960). She seems to have turned out reasonably well as an adult, but I don't know enough about her to say.
Amanda Bynes on Nickelodeon. I don't even know if she played a character and if so what the name was, but she was just gorgeous at that age. She seems to have run into some problems as an adult, and I hope she is able to work that out.
Those are just a few that come up off the top of my head. There were many more.