Sounds like things like beat the buzzer could work here, particularly if you combine it with a star chart and small rewards. I'd suggest maybe the initial cleanup being something you do together as an activity otherwise it all gets a bit overwhelming especially at such young ages.

I've also used the "if you don't pack your things away I'll give them to the Salvos" - only had to carry through with it once, and DD learned that it was easier to pack up when asked to.