I would go with the stickers.
i have stopped asking questions if there is a possibility that I will not like the answer when it comes to things like this. I ask parents if they have any allergies or other things I should know about and then leave it at that. For gift bags the parents can intercept and remove what they want to.
Several parents from daycare were talking about parties, gift bags, and presents. From the 15 minute conversation I discovered that each house has a different thing they don't like. Barbie dolls, wizards, harry potter, corn syrup but not sugar, sugar, apple juice but other juice is ok, cake, ice cream, temp tattoos, make up, Bratz dolls, Monster high dolls, religious toys and music, music that is not religious, any toy that resembles a gun, shirts without sleeves, two piece bathing suits, co-Ed parties. Disney movies, anything that is not a Disney movie.
Now I let them know the party theme, ask about medical conditions, and hand out gift bags when they leave. The parent can decide not to let the child come, and they can take away the gift bag,