Ask her what she finds interesting and based on her answer(s), find the most appealing to you and start designing a logic based on it, a prototype if you will.
Tell her that for this project she will be your project manager that would lead to the fruition of the end product; that would be enough to keep her engaged with the whole process.
Our son was very fond of acting as a "project manager" when he was young - like 3 or 4 years old.
His idea of playing with a (age appropriate) train set was to get his mother to set everything up under his direction, similarly going to the beach he'd direct me to build a dam... (which I was happy to do).
Tell her that for this project she will be your project manager that would lead to the fruition of the end product; that would be enough to keep her engaged with the whole process.
Good luck!