I hate to be the bearer of bad news, but rubbings simply take longer to dry. I don’t know of any way to make them dry quickly.
If the rubbings are light (which I don’t expect if you are working with kids) AND made with IRIDESCENT colors, you can turn them paint side down on baking parchment and heat set with an iron right away. The paint won’t be totally dry, but a film will form over the top.
If the person making the rubbing has a heavy hand (most of us to begin with and most kids), there is simply too much paint to have much luck with quick heat setting.
If you are working with MATTE colors, be very cautious about trying to heat set the paint without at least 24 hours of dry time. Several of my experienced customers have had problems with discoloration when they tried to push the heat setting too quickly with matte colors.
You might try a heat gun (not a hair dryer) on a few samples and see if you have any luck getting the paints to film over without discoloring. Definitely test before you plan on using this for your class.
If all else fails, lay a piece of muslin or paper towels over the painted fabric, gently roll or fold it up, and send it home with instructions to set it aside for at least a week. If these are not going to be washed, they don’t actually need to be heat set. The paint will dry on its own over time.
