I wonder how many people know this museum by its proper name. It's quite long.
The museum is very old-school in its exhibit style. There isn't much interactivity, instead focusing on showing what a lot of animals look like. There are descriptions and details in Japanese, I'm sure with some explanations about behavior, location, etc, but really this feels like a "let's look at stuffed animals" museum.
I recommend the museum for parents with kids who love animals, or for true biology fans. The National Museum of Nature and Science has great exhibits, but it certainly doesn't match the current preference for museums that include interactivity. Plus, there doesn't seem to be any description of how the environment and all these creatures work together to create an ecosystem. For active children, you may want to skip this one all together and head to some of the more-interactive science museums, like Miraikan or Science Museum Tokyo.
Ueno's science museum is open 9-5 (8PM on Fridays). As with most Japan museums, it's closed Monday (or Tuesday if Monday is a national holiday) and December 28-January 1. Admission is 600 yen. You can find it in Ueno Park, on the east side across from the zoo. Most visitors coming from Ueno Station will probably see the giant whale out front.