As most pp have said, I don't see the harm in it being a treat.
Last night DH and I were trying to decide what to do for dinner, after an awful week neither of us had the energy to cook a big meal. I didn't really want them eating takeaway tho, so the quick meal alternative in our house are toasted sammiches or toast with a side.of tomato and cucumber
And when we do allow the girls to have maccas I let them have the happy meal wih lemonade. In for a penny, in for a pound!