We just have a celebration for my 2 kids who are on same birthday on last sat. We stayed one night there and also had buffet dinner in the hotel restaurant. If you pay extra $ for a cake, a character person will come to sing a birthday song for your kid. Mine was Minnie (my girl like it). You can choose either having buffet dinner/lunch for your kid.
However, if you made a big party, last year I checked should be $300 per head and min is 60 person. But not including character person, you have to pay them in extra.