I wouldn't say this as stabbing as parents also looking after their interest. I have no idea what you should do as I'm not familiar with day home care rules here in Canada tho I'm learning. What I can suggest you to advertise for March and if they do come back and you have space then take them back. I know it won't help in this case but if you put month notice in your contract and take 2 weeks deposit beginning of the care and not to use that deposit for first 2 weeks care. And yes you should make her pay for that 2 weeks.

Hope it helps.
Cocoon