For decorating cookies, I wouldn't use buttercream. It doesn't set up firm enough.
I've given piping bags of royal icing to kids and it worked fine. Some of them are better than others at keeping the bag from "backing up" and icing coming out of the top, so if I were going to do it frequently I would definitely use rubber bands or those special bag-closer dealies to help with that.
Use meringue powder rather than raw egg white to make your RI and there's no problem with licking fingers.
Another possibility is to use egg-yolk paints, or piped dough, if you can decorate the cookies before they are baked. Egg paints are just egg yolk mixed with food coloring, and you paint it onto the cookies with a brush. Colored piped dough on a dark gingerbread cookie comes out wonderfully! Use the same sort of soft dough you'd put in a cookie press for piping.
Or forget the concept of decorating the cookies by putting something on top, and just make colored, hand molded cookies.
Like those candy cane cookies made by twisting a red and a white "snake" of sugar cookie dough together. Use a sturdy dough, offer several colors, and let the kids sculpt with it. No icing necessary.
No matter what you use, little kids will tend to pile on way too much
and make a mess, but they have a lot of fun with it anyway. Older kids (past the preschool stage) can be very artistic with it.