I think it's a great idea, if you are willing to do it. Me, I am not willing! lol
Seriously, I think it's a good way to offer something that not all providers offer. I would just think through ALL the logistics. Will you take a quick break in between your regular dayhome hours or do a continual care type thing? Personally, I would get parents to pick up, have dinner and bring their kiddos back in pajamas so we could have a jammies and movie/ dance party. Will you feed them dinner otherwise? (I probably would, to avoid moms and dads bringing dinner) How late are you willing to have them at your home? (I would say midnight) Will you do bath time routine? (I wouldn't)
As for how much, I would say whatever your regular daytime rate is. Possibly a little more, since it's night time? And I think I would offer it as a trial thing, so that if it's not your thing after all, you don't have to continue. Once every 6 weeks should be a good timeframe, or you could offer it to parents on an as needed basis?