Don't want to be over capacity however don't want to get screwed either

[Fun&care]

If I were you I would just wait until December until you say anything to the part time family, because so much can happen until then. Especially if you suspect that another family might leave anyways. And if the part time family doesn't like the option of coming Fridays then too bad IMO. Full timers get priority around here, and she is on mat leave anyways so it's not going to be the end of the world. I think if you wait til December and give them one month notice that your schedule is changing that's fair.