She needs to sign the contract now and pay the deposit to hold the spaces - deposit for both that is non refundable and goes towards first, last whatever weeks you have. I would say she somewhat forfeited her first deposit if she made one when she switched to PT and brought your income down.

This is the same as if you did't take them and interviewed next week and signed on a new family. Either one could just as easily stiff you in September with no job and pulling - moms on mat leave are supposed to have a job to go back to but it doesn't have to be the same one so some don't like what is offered and don't take it.