Hi, Melanie!
I just double-checked the scheduled park hours and I'm also seeing Extra Magic Hours from 9:30-11:30 PM at Disney's Hollywood Studios on Monday, March 31, 2014. I wholeheartedly agree that 9:30 PM is really late for most little ones to be able to enjoy the
Disney Junior Dance Party. Unfortunately, this event is only offered during Evening Extra Magic Hours, so I don't think there's any chance of it starting before 9:30 PM.
The Disney Junior Dance Party is a fun event, though, and there are a number of Disney Characters who come out to dance and interact with guests in Animation Courtyard. If you would like your lil' Mouseketeers to experience the Disney Junior Dance Party, my best advice is to plan a leisurely morning followed by an extended midday break back at your resort hotel with sufficient quiet time for a good nap. This afternoon downtime may help to stave off a meltdown later that evening when trying to extend their regular bedtime ~ and taking a rest will hopefully re-energize your little ones so they can show off their dance moves with some of their favorite Disney Junior friends!
I hope this helps! Please visit the Moms Panel again with any additional questions as you continue planning your vacation.
Have a magical day!