Have you considered Pulp?

https://play.date/dev/

I’ve dabbled with a few game engines myself, but I feel like Unity and Unreal Engine have too many features, which actually makes them less beginner-friendly. It’s hard to know where to even start.

For pixel art games, I personally recommend GameMaker. It specializes in 2D, and since hits like Undertale were built with it, you can definitely create "serious" games. GML (GameMaker Language) is also quite easy to write, so I think it’s great for learning algorithms.

You don't say if she can code.

I recommend Unity.

Unity is not that difficult if you clearly understand the Template Method pattern. The difficult part is that students often struggle at a different point than expected.

That said, if not Unity, GameMaker is also a good option.