sunowner, alas the whale has gone! We visited in February and I had told my little one all about it and we were both disappointed when we discovered it was a whale no more!
There is something for children of all ages at both the NHM and SM and there are as many buttons to press in NHM as in the SM.
Launch pad is fantastic but always always extremely busy and hot. Same with the dinosaurs in NHM.
My top tip for any museum would be use the cloakrooms. Drop everything off apart from a bottle of water so that hands are free and shoulders don't get tired.
mrsofty what sort of tips do you want for the underground? It's easy to use although it is a reasonable walk in the tunnels from the station to the museums for little legs. Mind you the children enjoy the opportunity to run in safety and make a lot of echoing noise!
The museums are always busy even in term time because of school visits. However there are always quiet spots and there are plenty of benches to sit down and catch your breath. One of the quieter areas in the SM is the top two floors which are taken up with exhibitions on early medicine. There are some gruesome things up there like shrunken heads, elementary dental surgery weapons, oops I mean instruments, bones, mummies and so on. My daughter was only three when we showed her this the first time but it really caught her imagination and she absolutely adored it.
I love both museums and only wish that we still lived in London to take advantage of the free entry.