Depends on the child I guess.
I remember the NHM being the BEST THING EVER at that age (we're talking mid to late 90s here), whereas even later in life when I was actually interested in science and engineering I've always found the Science museum a bit dull.
I know somebody who's an accountant...
I've always thought the Natural History Museum is more exciting for children that age, because all the dinosaurs etc. look f***ing massive at that age. Whereas the science museum's a bit more "look at this fascinating bit of machinery from the 1930s".