Tough one!
Are you in Ontario? Will the Bill 10 I would think you will have no problem what so ever filling under 24months spaces so taking a financial hit for a year to fill the spot doesn't really do you that much good.
EXCEPT keeping the older sibling 3 days a week to have them full time when the baby starts (if you will have space) does mean you have a over 2 space filled which I suspect is harder to do now that there are so many more over 2 spots out there.
Can you function for a year losing 2 days pay? Was the family a family worth keeping? I would think there are many variables in answering this. The key thing would be whether you expect to have space for the younger sibling when the child turns 1. Does that space open right when needed or will it have sat empty for months prior. Will that family pay for it to secure it when it opens up?