At 11 years old any system can work already. If she is interested, D&D or any other mainstream system will work just fine. You just have to guide them through character creation more. Probably limit some choices or even come with pre-created character sheets they can choose from.

But if you want something simple the Core Micro system would work. It's free and you can download it on itch https://as-if.itch.io/core-micro The rules are very open and can be used for any setting. The stats determine how many dice you roll and you just give them the target number. Depending on how close to the target they are, they will succeed or fail with either something positive or negative happening on top of it.

With a little homebrew, you could even turn it into a Harry Potter setting. Just add an additional 7 attributes that are named after the Harry Potter spell types (Charm, Jinx, Hex, etc.). And then she can either use traditional attributes for checks or magic. Which has the added benefit of giving her always 2 chances to succeed. For example, she has to jump across a gap. If she fails her Grace check and is falling she can still make a Transfiguration spell check to see if she can build a bridge out of something nearby.
And the skill list is already intended to be expanded upon, so you can add stuff like Quidditch, Potion Brewing, or Rune studies.