Áine, with 1st class you can be quite open on how they might want to make their rocket, and there could be some great discussions on fair testing if they wanted to compare their rockets with each other. I have made the foam and rubber band rockets with 1st class, with a few helping adults.
Or, you could demo one of the water rockets that use bicycle pumps as an exciting prompt.
There is a school in Galway that launches a rocket a day during Space Week.
You can see some examples here.
http://junior.renmoreschool.com/News/Making-and-Flying-Foam-Rockets-(Projectiles)/21203/Index.html
http://junior.renmoreschool.com/News/Fizzy-Rockets/22880/Index.html
http://junior.renmoreschool.com/News/Balloon-Rockets-(Projectiles)/23654/Index.html
http://junior.renmoreschool.com/News/Using-2D-and-3D-shapes-to-make-air-rockets/21977/Index.html
http://junior.renmoreschool.com/News/Space-Week-Rockets/24428/Index.html