Why not tell the parent that you don't normally take SA kids so you have nothing for her planned so she needs to bring some books, some paper and perhaps a small toy activity that she could entertain herself with while there.
You could give her a play space away from the others (at the table even) and leave her be.
Honestly, I wouldn't have agreed to take her at all.
I've been in the "just this once" situation before as well as "I could really use the extra $" situation too but if I am honest I never once ever felt it was actually worth it.
It's easy to say yes when asked but I almost always regret it. Well not almost always but always. *sigh*
@Storybookending....yes! Plowed paths for the little ones is awesome and fun! They love that they aren't confined to the sidewalk and most little ones have a tough time walking in snow gear anyways so the plowed paths are a ton of fun!