
How to Reassess Your Chess & The workbook
Complete Book to Chess Strategy
are both good. They might be a little heavy for a 7 year old but with some guidance I think they're doable. I think I read them when I was 9 or 10 and didn't have any trouble.
Chess.com lessons are really good too and might even be a better place to start.
chess master 10 had a great walkthrough tutorial on strategy by josh watzkin that i did around that age, if you can find a copy of it