Er, if you say classes two times a week, does this mean
a) a total of two classes, both in one week?
b) two classes per week for a longer period of time?
It'd would make a considerable difference 
But I suppose it is a total of 2 classes, each an hour long for now. My first advise would be *Run, Forrest, run!* and try to get out of this if still possible. If you have no experience it might be hard to start with children. Especially if the time is also so short. One hour seems nowhere near enough time... and if all this was not enough, you also have a theme to go along.
OK, some suggestions.
The kids are old enough not to make a complete mess about this, that's good. They should be able to handle 3-6 cookies each (depending on cookie size and complexity).
I' d give them 3 plain cookies they can ice during the class. But as the icing will not be dry enough to do much detailing, how about preparing an additional 3 cookies each which you already basecoat in advance. You can then teach them coating as well as piping details with stiff consistency.
I have no idea what to charge for it, but around where I live those kind of classes for kids start at appr. 100 USD. But limited to 8 children max.
Good luck 
Hello Laegwen
The classes will be Monday and Wednesday for 3 weeks six classes in total.
I like your idea to give them cookies already base coated and maybe for
decoration edible markers, sprinkles with small amount of royal icing.
Thank you very much I really appreciate your comment and suggestion.