Niece wants my help with a soil sensor for her class 8 science fair. How much is too much?

She has watched me fuss over the terrace garden for years and decided her project will be a little sensor that tells you when a plant needs water. Her parents happily handed this one to me, since I'm the 'tech aunt' of the family. I could build it in one evening. That's the problem. I want her to do the wiring and the explaining herself, but she's 13, gets frustrated quickly, and the fair is three weeks away. Where do you stop helping and start doing it for them? ❤️