I'm doing a school project with 4 year olds and per school policy can only use store bought products. I need the frosting to dry quickly and firm up stiff, but edible. Do any of you know if I can add meringue powder to the store bought tub of frosting and have it behave in the manner I need?

If so, how much do I add to one tub?

Thanks in advance.

Simple answer: Yes.
Next ? : Why? MP is NOT what makes icing 'set up'......it's the amount of sugar to fat that causes it to crust. Thicken it w/powdered sugar.

It sounds like you are wanting it to get firm like in RI? Not just crust like buttercream? I have no idea if that would work though.

MP is NOT added to RI for stiffness. RI is egg white + powdered sugar. Meringue powder is dried egg whites plus cream of tartar (and sometimes some other ingred to help w/ the stabilization of the egg whites), you mix it with water and use as a replacement for fresh egg whites in the RI recipe.

I would add powder sugar or cornstarch to the icing.

Have you thought about using Wilton's icing in the tub? It is a little thicker than the regular store bought and will crust over slightly.

Also will they let you use a powder mix that you add water too? If so then Wilton also has some royal icing mix for their gingerbread houses.

Or another option would be fondant.

Your best bet is to use Wilton's small cans of white decorator icing. It is stiffer already.