I agree with the rice krispy treats and fondant. How much are the supplies going to cost? How much are you paying yourself per hour? How many hours do you think it will take you to make the cake, frosting, decorations, etc.? Answer all of these questions and you should get a fair price. I do not think it matters whether it is a child's cake or not. If they want that cake then you tell them how much you can do it for. They either choose to accept the price or not. I always ask what the customer's budget is even if they have a specific cake like that in mind. Then I try to still give them what they want (maybe scaled back a bit or more) and still be true to myself as an artist and baker. I work hard and caking is not easy. I hope you are able to do it. My oldest daughter would love that cake just because of the minions. Good luck.