Sharing a space sounds like a plan!

And yes you ARE the one who in control here hon you just need to REMIND them of that by letting them know the service options open to them and the date you need them to confirm that option to accept by or the contract will terminate

With sharing the space ~ I am assuming that means you will fill your remaining ones if possible to keep your income viable ~ so what happens at the end of the maternity leaves if they both suddenly want full time for 2 children each ~ will you have FOUR fulltime spots for them to return to??? If not you need to make that clear in any contract you sign that them continuing to send their child over maternity leave does not mean that full time spaces will be available upon return