It's interesting I did something similar for a local sport club. A couple of suggestions (which was something a full time dance photographer suggested to me) was get your conveyor belt set up (I'd say an hour per person is way too long) shoot three or four poses and then charge for providing eg two high res images (not prints) maybe £50. This is just an example, you need to flex the variables to come up with an amount you are happy with and that is attractive to parents to purchase.
The other model is to work out how many people want their photo taken and quote a total amount to be split between all parents for x many images per child. That way you are definitely getting your money rather than hoping a parent will pay for the images, it's amazing how many all of a sudden change their minds. (So if you are going down the pay per image route as much as I dislike watermarks generally, put a dirty great "draft" across the image, although with AI removal even that isn't the deterrent it used to be)