I have a cake order but the lady says her kids are allergic to buttercream and asked if I could icing the cake and cupcakes with store bought icing like bettycrocker. I personally was shocked that she asked me to make a cake and then asked me to icing the cakes with store bought icing.

My question is what other icing would you use on a chcoclate cake with banana cream filling and a white cake with vanilla cream filling? I was thinking a cream cheese icing but not sure it would taste good together. What are your thoughts?

What part of "buttercream" are they allergic to?  There are a variety of icings that you could make, but I think you would need to know what they're actually allergic to before you could make an informed choice.

Is it dairy they can't have?

i would push her to tell you what exactly the "allergy" is as far as ingredients go and how severe the allergy is.  It really seems like she is using the word "allergy" to mean preference (something that is really annoying to moms like me who actually have to deal with real allergies with their children). As in "I'm allergic to doing dishes." 

If there is a true allergy, then you need to know what it is so that you can review all of your ingredients in your recipes, and see if you should even take the order. For example there could be an issue with cross contamination with your equipment if the allergy is to nuts and you routinely bake with nuts on the same equipment. When dealing with allergy sensitive clients, you also need to have a signed contract alerting them to the fact that while best practices are used to avoid cross contamination, that you there may be trace allergens present due to supplier manufacturing and use in your kitchen. 

Also, if you have professional standards that won't let you in good conscience present a cake with your name on it to the public that is covered in can frosting, then don't.  I can't think of one (reasonable) situation where I would agree to go against my signature recipes and serve what I feel is an inferior product just to make a sale. What happens if someone at that party thinks that you are the cake decorator who just uses can frosting?  That would could kill future orders.  Just some food for thought there on how to protect your image. 

Thanks for all the information and suggestions. The lady is not sure just that their skin breaks out when they eat butter cream. I will have to look into other frostings I could use that will taste good with the flavors she choose.

Maybe the canned stuff if all her kids will eat.  And you can't really decorate with that stuff, can you?

Depending on my mood, I may offer to bake the cake and she can frost it herself.  But then again, probably not....

The cooked flour frosting is pretty good. A touch of meringue powder will help stabilize it. Everyone who tries it loves it. You may want to give it a try. 

A"So sorry, I cannot accommodate customers with food allergies. Baker X in my area is a great baker who does allergen-friendly baking."

If this is a true allergy, they will know precisely what it is. If this is a lady whose kids like Betty Crocker in a can and doesn't feel like frosting a cake herself, she can take her business elsewhere. IMHO.

The others have a good point.  You can't toss around a word like Allergic, but not know exactly what they're Allergic to.  Who knows, maybe they are a hot mess after eating sugar which makes her break out in hives?  But listen to the ladies who have uneasy feelings about this woman.  All she has to say to a court is that she warned you that the kids have allergies and then they broke out so you didn't follow her instructions.  Boom........you're out of business or out of an insurance company because they had to pay out a claim against you.  Get rid of her. 

saying that you are allergic to buttercream is like saying you're allergic to soup.  

I'd steer clear and become booked for this order.  Unless you want to be known as the cake lady who uses can frosting. 

Thanks for all the comments and advice. The lady ended up cancelling the party due to weather and I have since told her that I cannot make this cake. I don't like using store bought stuff to make my cakes and the whole "allergic" thing always bothered me. So it worked out well.