If you do take them on just have them leave a few emergency foods at your house. Something with a long shelf life that you can use to top up the child's meals when needed or in a pinch feed the child should they forget the child's meals.

As mentioned above I would ensure that the child is ok if there is minor cross contamination on the rare occasion. I would think with gluten allergy it is ok...as it typically just makes them sick to the stomach (though with potential long term consequences) and not anaphylactic.