Kids between 7 and 9 can start to enjoy the wonderful world of Harry Potter. Keeping in mind that all readers and kids have different developmental stages and some on the younger side would do better reading these books together. For this age group reading up to Harry Potter and the Chamber of Secrets.
With kids age 10 and 11 the movies get darker (the books get more intense and beloved characters die). So with that being said children in that age group can typically enjoy up to the Harry Potter and the Half-Blood Prince.
Once your child turns 12 they can handle pretty much anything in the Harry Potter World even the very intense last book
This series is such a magical ride and being able to read the books and watch the movies together as a family can be such a joyful experience for the whole family.