Visiting Houston and Galveston with kids and in-laws for 3-4 days in December

I will be visiting Houston with kids ( elder 6 years old and a 7 months old ) and in-laws in their early 70s.

What do you recommend we should do – places to visit and things to do? How much time to spend?

My 6-year-old is very interested in any animals, loves to read books, and space-related stuff (of course we will visit NASA).