If its just a one off, i wouldn't have a problem with it. When i was a kid every now and then for school excursions we would go to macdonalds. It was a fun treat. Hardly any kids were fat mostly because macca's was reserved for occasional birthday parties or car trips. We didn't have take away 3 times a week or play the play station 4 hrs a night….
If your kids are healthy and active, then i wouldn't think one happy meal would really do any harm.