Unless you will have a space for both children when the mat leave is over it is best to just cut ties and say goodbye. By a space I mean you have another child leaving to start school, start mat leave, etc and you know it will be open by the time the mom needs it without having to save it for more than 3-4 months.

Otherwise I would be filling the space you have now and let the chips fall where they may a year from now. The older child staying with you is for the mom's convenience not about holding her daycare space so once she gets a routine down at home - ie 6 weeks she will start to reduce days so remember that it will be about holding two spaces not one. I would only consider holding a space that long if the parent paid full time for the older child's space whether he came or not and paid a holding fee for the other space. Once you sit down and put the amount you will earn per month down on paper if you replace at your current full time rates and do the same with what you will earn for a part time child plus a holding fee you will be out at least the total of one full time child's income for the entire year - way too much money to lose.

And even then things change - family decides they need a bigger house and move out of area, mom decides not to go back to work or is given a different position that means hours that no longer work with your hours so too much of a risk.