Aoife,
glad to know that the mice have been a hit -I do just love them.
I really like how you have planned for lots of discussion around:
we could discuss if there were ways to improve their rockets, what might they add to it or remove? How could they get it to go faster or slower? What makes the rocket stay on course? What propels their rockets? Could this be changed?
Somewhere I used to have a link to a video which showed a family doing rocket races and they found that the larger balloon was slower to get going, but went further, and the smaller balloon zipped off first but then ran out of air and stopped – so you could challenge your early finishers to test the effect of distance of race and how it interacts with size of balloon for determining who wins the race. Might include a mention of hare and tortoise!