how long does it take for the plaster to dry? I do event stuff and I have a client that wants me to do something like this at a 4 hour event.
it only took an hour or so to be dry enough to peel the clay off though the plaques were about 1.5cm thick and it will be very soft when sanding the edges so be gentle and use 1200 grit paper! if you do the plaster thicker it will take longer but too thin and its going to have no strength. i wanted something you can pick up and handle
it can take days to dry properly. you will really notice the difference in the weight but it should be do-able in less than 4 hours.
i would recommend telling the parents to leave it somewhere warm when they get home for a few days, keep the handling to a minimum until its light and dry as it can be scratched easily at this stage...and get hold of plenty of bubble wrap for safe transport