I'd say either get payment on a month-to-month basis (in an amount you're comfortable with- I wouldn't do less than 1/2 rate), or tell them they're taking their chances.

I recently held a spot for 5 months with only a deposit... Because I'd have an opening in that age group when they would join. Turns out the child we were waiting to 'graduate' isn't with me anymore... So now I can't fill one of the precious under 2 spots, but still not getting any money. I will not do that again!