I went to Valencia last year, end of July and beginning of August, it was hot but it's a fabulous city, lovely sandy beach within city, great museum area, which is air conditioned, aquarium is great. I've heard the zoo is great as well but we didn't get there.
There's a lot of trees and a big park that is in the old riverbed, goes right through the centre of the city, and there's a great gulliver inspired playground in it, basically a massive man lying down that kids can climb on and slide down.
There's also the battle of the flowers festival last Sunday in July, which is great fun, at the end everyone is throwing flowers at each other.
I went with my 8 yr old son, and we loved it, very friendly city.
I think as long as your accommodation has air conditioning and you're happy to change timings of things, so don't try to do too much in the middle of the day, you'll have a great time.