The partner hotels (Kyriad, Explorers, Magic Circus...) are are connected with a free shuttle. You can save money booking from Expedia or on the hotel website.
At the RER Val d'Europe, there are an Etap (maybe this was renamed recently, I'm not sure) and an Ibis hotel. The Val d'Europe RER station is 5 minutes from the park (1 stop) and the Etap is right at the exit.
At Bussy St George (10 minutes, 2 stops) there are cheap but nice hotels: Formule 1 and Campanile. They are very cheap but it's a 10-15 minutes walk from the RER station. It might be too long after a day in the park with children.