I'm not in Ontario but I wouldn't do it for anything less than $100 a night. You'll need a clear contract regarding drop off and pick up times. You also need to think about that child being gone before your day care opens in the morning or the overnight child will be infringing on a space when business picks up. i.e. If you open at 7am and that overnight child is still there at 8am, you wouldn't be able to take your full quota on so they need to pay for the day care space for the following morning too.

Think very hard before you do this. Effectively you'll be on the clock 24 hours if you do have day care children too. While I understand it might be tempted - and short term - you have to decide if it's really worth it.