Read first, or watch first?
No film of Tom Sawyer has ever become the classic the book is, which simplifies your decision: the book is the main event, and it sells itself one scene at a time. It belongs to grade 5 and up, because Twain's 1870s vocabulary asks something of a reader. If the family wants a movie night anyway, Tom and Huck afterward is fine, and read the content note first; the book's 1876 attitudes need the same two-minute conversation you'd have before Huck Finn.