Went back in December with my nieces and family. Was incredibly expensive. 3 year old is fine but 18 months - no no. Ideally you would want them to be about 8 years old as they would be tall enough to go on the big rides. My 8 year old niece was able to go on all the rides such as the Terror Tower (which was frickin awesome) and the other roller coaster, Aerosmith thing.
We all stayed in the New York Hotel which was a complete rip off. It's about 5 minute walk to the Disney Cafe or whatever area is with all the food and coffee shops. Then another 5 minute walk to the entrances to both Disney and Universal. The hotel is run-down and has very very rude staff. Made us wait nearly 2 hours to check-in, promised us an upgrade but instead put us in a room with the bath missing 10's of tiles and pretty much in pieces. Bed's were big and uncomfortable, but the Food was brilliant.
DON'T GET THE FOOD VOUCHES IF OFFERED TO YOU WHEN YOU BOOK YOUR TRIP. They are a COMPLETE Rip-Off. There were so many rules and restrictions it was stupid.
Unfortunately for us, Space Mountain was closed! Darnit!
Don't forget to get Priority tickets throughout the day. I guess you wont be spending much time in Universal as it's not very interesting for children of 5 years and below!!
Still, The Disney park is great for the younguns. They do a fantastic parade type with lots of moats etc. Don't know what the word is. Complete NIGHTMARE if you get want to leave the park while it is going on, they close off literally all the paths to the exit!
Overall, Would I recommend it for a long weekend? Meh, Yes. You really want to wait until your kids are about 10-13 before going to America for the "Real Disney".
You can find good deals if you look around.