It is super easy to get to from Ueno station; just take the park exit and walk through the lovely park.
Entry was 6.10yen per adult, and children under 18 (yes, 18!) are free. It was super cheap!
There are elevators but it isn't super stroller friendly. It is much easier to access things through the escalators. However, if you don't mind waiting for the elevators then it isn't much of a problem. The layout of the museum was super confusing. You enter in B1, and then work your way up or down, and there are kind of two buildings (most of the exhibits are in the back building).
I was actually a little disappointed by the museum. I have been to some of the bigger ones around the world, and was expecting something much better than it actually was. I think the museum in Brisbane (my home town) is actually bigger, and better.
They have a small dinosaur exhibit, and some taxidermied animals. They have some science-y type exhibits that are mostly Japan related (like an earthquake simulator). They have a few other exhibits too, but nothing that made me go 'wow!'.
On the third floor, there is a kid's playroom called "Compass", which is free to get into, but needs to be booked ahead through their ticketing system. We got into the 12:00-12:45 timeslot. Pi had fun in there, which is mainly a big junglegym with taxidermied animals hidden in there, and some activites. There is a T-Rex statue in there too. You get the ticket for Compass on the first floor near the information desk (not to be confused with the B1 floor information desk). I believe it is for kids up until age 13. Children who cannot walk/climb will probably not enjoy it. Best ages for kids is 4-6. Pi enjoyed the run around though.
They have a cafe with some free space where you can bring and eat your own lunch near the B1 entrance. There is a restaurant there too (I think on the 1st floor), but we only had a glance at the menu. The prices were okay, but I can't comment on the taste. We ended up leaving at about lunch time and getting something from Ueno station that was most to our liking.
There is a gift shop too with some really cool things, and a theater with some sort of movie showing- but we didn't watch it.
Even though I was disappointed in it (because of my high expectations), for the price I definitely recommend it. It was a nice way to spend a family day. I hear it can get super packed on public holidays, and rainy weekends though. I thought it was busy enough on a Friday with all the school groups.
More information can be found in English via their website:
http://www.kahaku.go.jp/english/#