to really get your money's worth from the park, i would leave it for another year or two. There are plenty of rides they could go on but to be honest, they're the kind of rides you could easily find to go on in the UK.
Also, the firework display over the castle is very, very late and for very little children, it can be too much too late and not the charming experience you might hope for.
If you've got the dosh, stay in the Disney Hotel itself - straight in and out of the park with no walking/buses to mither about.
My boy was 3 and there were lots of rides for him, but he's a little geezer who isn't scared of anything. In Fantasyland rides that you sit and just go slowly are OK apart from snow white because of the witch (my boy loved it it depends on the child). There are a few boat rides that would be fine. They'd love the characters. I agree with staying in a Disney hotel if poss we stayed at Cheyenne and it was great apart from over crowded breakfast times.