The great thing about all principal London museums is the free admission, though there are always some temporary exhibitions that are not free and you are always welcomed to donate to the museum any amount of money you wish. Another amazing point not only for little ones is interactivity.
Once you cross the museum threshold you realize that it is definitely the perfect place for boys. The first show room intimidated me as a girl: all sizes of steam engines - monstrously big ones look like polished monsters that can devour you, and toy size tiny ones that remind you childhood. Personally I didn’t get any of this stuff but my husband was very astonished. Although we didn’t have enough time to see all the exhibition rooms, the museum impressed us. The aerospace room contains everything concerning this area: from engines, flying machines to the astronauts food - no need to say what interested me the most =) Then was the room dedicated to the science evolution by epochs till our days. You can touch many exhibit items, far many things are not in their original size as some of them are really gigantic and this gives an impression that you are in a toy store. There is a great exhibition of the history of watch, which is really amazing, I mean, everything before the electronic watch was invented. There are lots of things above that, also many activities for kids and stunning gift shop where I forced my husband not to buy every second thing. I reckon the Science museum is actually worth to be visited especially if you are or have a child or husband and you should give it not less than 4 hours.