Muireann,
with an older class the children can watch the demo, then try out their own ideas about making the rocket go fast/ carry more.
I’d structure it slightly differently, by giving the children their own balloon first, so they can see that it flies off when released – then ask them how can we make this balloon become a rocket? This then gets them to the balloon rocket set up as a ‘design and make’ a launch system for a balloon, and they have more ownership over the activity.
There is some really lovely science that can be deeply explored, particularly if they try different size balloons and different amounts of inflation. Get a bunch of balloon pumps!