I advertised for a year on cookie decorating parties and never got a hit, decided to remove it from my website, then went to a party and talked to a young teen and she almost hit the roof when I told her I could come to her bday party and show her and her friends how to decorate cookies. Go figure!
When I was advertising it on my webiste, I was charging $10 per child with a minimum of 8 children, and that included four cookies, a take-home box, a gift certificate for each child toward a future purpose, a custom cookie that I would decorate for each child ahead of time (so the parents could see what I was about), and sprinkles, fondant cut-outs to use as decoration, three colors of icing, etc. This did not include me, just a box of stuff that they pick up. I think I would have to add like $15-20 per hour for my time if they wanted me to be there to help.
I had a cake decorating party for my daughter this past year. she was turning 7. There were 12 girls and it was a blast!!! On the invitation I gave each girl a choice of a flower shaped cake or a butterfly shaped cake.(I used the wilton pans) I had each cake ready for them when they got there. I iced only the sides of the cakes so they could decorate the top. I made all of the icing colors in advance and had them in bags with tips ready to go. I had 3-4 of each color and it went very smoothly. I had plain white cake boxes for each girl to decorate when they arrived before doing the cakes. They used stickers, foam pieces, markers, etc. and they did that for about 1/2 hour. Then they decorated!!! I showed them the basics with the tips they were using and let them go. They had a great time and the cakes turned out great! They were all very proud of their work!! I did have the 2 teenage girls that live next door help me and that was a great idea. They helped the girls and cleaned up after!!! I would do this again for my other daughter when she is the right age. People are still talking about what a great party it was!!!
If I was to do this and charge, I will say it was a lot of prep work on my part so take that into consideration when coming up with your prices. I I would have to atleast say $20 per child, if not more. Maybe you could do a per child price plus a prep fee.
I have cake decorating parties advertised on my website but I haven't had any replies yet. I am hoping that if I can do just one then it will catch on. Looks like fun!! 
I do cake decorating parties. I charge $125 for 10 kids, with each additional kid $10. For that, they get ten 4"cakes, 10 bags of colors (3 colors total) and cake decorations. They can either decorate the boxes at the party or I pre- label them for each kid. Each child gets a disposable apron to wear also. So far it has worked really well, and the kids have a lot of fun.
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