I have had this happen to me ... I knew I would be loosing a child to school full time as well because they were not in my school zone ... the client send the older sibling 3 days a week through their mat leave to secure that child's spot - I had a part time family who picked up the other two days so I was not out the income. For the 'new babe' I had the mat leave parent sign a contract at the beginning of her mat leave to secure the second spot and pay the security deposit on the space for a 'tentative' start date of X.

In hindsight if I ever do this again I would also added an 'amendment' that IF the space unexpectedly became available earlier than X date that to continue to secure the spot for X date the client would have to pay for Z # of days per week until X date or forfeit the spot and the security deposit would be returned to her'.

Z days being however many days you could afford to loose in income to 'meet them in the middle' so to speak.

Cause with me initially my child leaving had gave a tentative date of September 4th to leave ... they planned to stay for the summer but than got offered a before and after school spot attached to their school for end of JUNE where they could come full time for the summer in order to secure the before and after school spot come the September if they did not take it there was no guarantee another would come for September - so they pulled early on me which I totally empathize with but I ended up having an opening for 6 weeks between their 'security deposit ran out' and when the newbie was supposed to start ... now I chose to just let it go and forgo the income because I did not WANT a new babe starting over the summer because I like to TRAVEL in summer to keep cool and have fun summer outings and I knew if I had asked this particular family to pay for X number of days they would have sent the teeny tiny babe to me for the whole summer even though they were both home on leave

But in hindsight having it CLEAR in the contract of a prebooking that if the space comes available earlier they would have to either commit to that 'new' space or forgo the space due to the unexpected change would be prudent for me!