I would stick to my contract and only terminate care if a breach of contract occurred.

However, I would consider changing my contract to say something along the lines of "a part time space may be in jeopardy if a full time space needs to be opened. The part time space holder will have the opportunity to go full time but if they decide not too,the part time space will be terminated"

I actually think I'm going to change my own contract to include this....at the rate I'm going, I will always have part time children unless I do something like this to change it. Being limited to numbers, hopefully most parents would understand the reasoning behind this decision.