Rats are the absolute best pet ever and great for children. I have the Savic Freddy 2 Max from pets at home. It's a great cage with a really good size door which is handy for hanging hammocks etc up. I have 4 girls in this cage who fit very comfortably. I think it would be good for up to 5 girls or 3 or 4 boys.
I chose girls because they are a bit smaller than boys and a bit more active. My 4 each have different personalities. I have 1 that is super active, 2 that are more cuddly and 1 that likes to snooze. I would recommend going to a reputable breeder for your first rats and handle the babies and choose by personality, that's what I did and they've all ended how I expected.
They absolutely need time out every day for at least an hour I would say. Mine come out on the bed (with a throw because they pee on everything) each night and love running around, climbing all over me and having a snuggle. They are super easy to handle so when it's time for them to go back I just say 'home time' and them jump up my sleeve/on my shoulder and sit there until we get downstairs to their cage. They love sitting on my pockets when I'm wearing jumpers.
Rat Rations is a great website to get hammocks and food, just FYI.