Extra simple and perfect for a one shot or two: Definitely Wizards
Not quite as simple but I adore this game (free demo on the page): Wanderhome
Edit: just found out Wanderhome is included in this huge TTRPG bundle right now.
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.
If you'd like to play in a Harry Potter like setting, check out Kids on Brooms.
Mausritter, Carin, and DURF would work well for this.
Mausritter is based on Into the Odd and features cute mice exploring a world of giant proportions.
Carin is also based on Into the Odd and is more straight forward fantasy.
DURF is really simple and fast. It's really quick to make a character and get to playing.
Mausritter is a mix of some easy systems, very "kid friendly", it actually won some awards because of it. The Tiny D6 systems are also good and easy to learn: two dice, if you have an attribute that helps, use three.
Mausritter is also great at getting people into the "old school" adventuring mindset. It's easy for people to get that they're a tiny moues, so they need to be careful, be clever, and run away from dangerous situations.
Plus it's got fantastic first and third party adventures to run.
Might not be sufficiently fantasy magic for the brief though?
That's a pretty good point. Time to read it again! (and this time I'll play it)
Tunnel Goons might be a good fit. Extremely easy to pick up and play, endlessly mod-able and flexible. No wizards or magic out of the box, but would be easy to hack in (could even steal from my next suggestions).
Maze Rats is another super simple, quick to pick up system. If you choose to have magic at character creation, you get to a roll a random spell name each day and then decide what it actually does when you go to cast it.
The Black Hack (or a physical copy) is a nice, rules light take on classic d20 fantasy adventuring, complete with the traditional classes of Warrior, Thief, Cleric, and Wizard. Traditional DnD-ish spell memorization and casting.
Macchiato Monsters is a more flexible, open ended game based on the above suggestion (with some Whitehack mixed in). More flexible magic where you can create your own spells to do whatever you can imagine. A bit more work on the GM's side (although I've actually written a little guide to help with that).
And if I can briefly self promo: Brighter Worlds (or the online SRD) is my own game, in part based on games like Cairn and Electric Bastionland, designed to have a simple core system and modular add-ons for each of the different character classes. It's got a whimsical, magical, vibe and a lot of different options for what players might want to be doing (including several different ways to engage with magic). Might have a few more moving parts than you're currently looking for though.
Maybe Fate Accelerated? You can jump right in, just pick a High Concept (What the character is mainly about), maybe a Trouble and approaches (how you approach problems) and leave the rest to fill in. You have Fate points, which you spend to improve rolls and which you get by getting into trouble / adventure. The players have more ability to affect the story than in D&D, and is usually easier to get into for beginners than for experienced rpg players - check Book of Hanz for tips.
The simplest TTRPG I've used is FATE. I've had very good sessions with this system and the simplicity of it encourages Role Playing over mechanical complexity/min-maxing.
Kids on Brooms is a variant of Kids on Bikes https://www.huntersentertainment.com/kidsonbikesrpg that's basically a very free-form RPG system. Kids on Bikes has a very Stranger Things feel, whereas Kids on Brooms is basically a Harry Potter RPG. It's much more rules light than D&D and is more of a cooperative story telling session than a strictly rules driven session.
Discussion of table top roleplaying games.