The 3y is my niece; no pushchair anymore. Think she would cope with the walk; is there anywhere on the route to stop mid way for a drink if she gets tired? To be fair she has walked further when we went on a country walk, but her dad did carry her a little way too - I wouldn't be able to carry her really though.
Hmm, I reckon it depends how far you're coming and how long you plan to spend there. They've got a lot of stuffed animals and birds which my dcs find fascinating but may not be to everyone's taste. There's also a small vivarium (frogs, snakes, lizards), a drawing/book area and an ancient Egyptian section, also a t-rex skeleton. It would fill a couple of hours but not much more than that.
They do have some good activities on at the weekend and school hols, should be on the website. Also a good cafe with nice cakes. If you were coming from a distance, you could make a full day of it by also going to the MOSI or the whitworth gallery (and the park round the back of it).