Katie,
so much scope for fair testing in the way that you have described this.
If the children explore: How does the size of a rocket affect its flight distance?
then they need to be careful to use the same size milk bottle (I’m not sure a milk carton would work – I’d think they’d get squeezed in and not re-form their initial shape) and give the same amount of squeeze.
The mice are fun when launched straight up, but if you want to look at the horizontal distance you may need to set up a pile of books per group so that they can tilt their bottles to launch the rocket mouse sideways.
Size of a rocket might be easier to do with the balloon rockets, since there, the size is actually linked to the propulsion, since a longer balloon has more air in it.