Hello MG
The shellac drys in a minute, the first coat will disappear quickly in the dried plaster, and to see the shine you will have to coat it about thee times.
The shellac is natural, it is made with bugs 
, I like it since it dry so fast, it will come in different weight, called 30 lb or 40 lb cut, and this has to do with the amount of dissolved bug material in a solvent, I have not seen it in powder form to be dissolved, all I can get is the already prepared mixture.
So You should be able to get the children to use it, also if it is hot where you are, by the time you finish the second side of your HHG, the first one will be dry, so basically you can shellac the whole device and keep on going for a second coat without stoping.
I am most interested in what the children will say 
IMPORTANT: after you use brushes with shellac, it will clean spotless with ammonia, even if you forget a brush and do it the day after
josh