I would recommend reading at least the first 3 books in order. They introduce the characters and the tree house and some important background knowledge. After that I don’t know that it matters a lot.
I would suggest Pinteresting activities that go with each book’s theme or time period. That will give you Science and History activities. For example Mummies in the Morning takes place in Ancient Egypt, so I would look for stuff on pyramids, mummification if that’s not too morbid for your child, cats, Cleopatra, etc. I think I have seen some Magic Tree House specific lesson plans on Pinterest as well, but not tons. You might also check Teachers Pay Teachers, but you’ll probably have the most luck piecing together activities related to book topics. Each book’s synopsis should give you enough information to get you started.