I would choose your theme then the music to fit. Also consider how much money/effort you/parents are willing to put into costumes when choosing your theme. I am out of touch with current children’s movies, but they often have catchy songs/themes that are age appropriate.
Since the weather is getting nice a summer/beach/sea theme could work if it has not already been done to death. 60's, 70's, 80's, future, Australia, a certain colour. Equally think about a prop that could be used to do different dance/acrobatics like suitcases, umbrellas, body boards, hay bales (messy) along with your more common ropes, balls, hoops etc...