This review is based on a visit with my family; 2 adults, 2 kids aged 3 and 6.
It's about a 30 minute drive from Calais and easily signposted. There's free car parking and a new visitor center with friendly staff. We started with the Planatarium which was great fun. We watched 15 minutes intro to the solar system in 3D, followed by a 3D movie on the history of rockets. The kids loved it; note: the solar system introduction is only in french. Headsets are provided for English for the main movie.
We then visited the Museum walking though large tunnels dug out of chalk, lots of historical information and eventually a lift up to the dome where they used to house the V5 rockets. The museum is great, and the kids will enjoy the various rocket exhibitions. However, a lot of the museum is dedicated to the memory of those who died in the way, particularly in the concentration camps. I found it very educational, and well presented (English descriptions against all pictures, and English headsets for the various displays). However, my children were too young to understand or to treat those parts of the museum with respectful silence.
It's a great find, and very happy we visited.