Museums don't just mount exhibitions. They also sponsor classes, workshops, musical performances, lectures, special family events, and tours to interesting places all over the world and your home town. They have summer day camps for every child's interests. Some museums offer junior docent programs where junior high and high school students are trained to be tour guides or work in other jobs in the museum. Some institutions offer a grandparent membership, which costs a little less than a family one, so grandparents can bring the kids.
Members enter free as often as they want.
Good museums change exhibits often, but kids love to view the same exhibits, finding different objects to focus on each time. Most museum activities involve hands-on experiences, or viewing actual objects. For most people, this beats reading about something, as in traditional schools. Museums are great places for inter-generational learning. Kids can share their knowledge with parents or grandparents, and the elders can share knowledge they've gained through experience.