Matthew,
which rocket would you think to use with your 1st class children? We have found that foam rockets stretch their fine motor skills, so making them is possible, but you may need additional older people with stronger hands to do some of the cutting.
There will also be at least two children per class who will cut three or five cuts in the end of the foam instead of four.
This could be a good opportunity to buddy up with an older class who can assist (and make their own rockets).
The simpler pop rockets with the fizzy tablet are more accessible and here the children may spend more time creating and decorating their rocket, knowing that the launch is relatively straight forward if you have a supply of new rocket bodies to be used.