Not a right fit.

[Bookworm]

I posted on Friday about my issue with one set of parents and their failure to follow drop off guidelines for half days (no nap and lunch eaten). I talked to the father at pick up and while I was professional in telling him how Fridays had to be from now on, I know he left a little upset with me.
Before this incident happened, I was having reservations about keeping them, and was planning on discussing with them whether or not they had planned on staying, but didn't because of what happened on Friday. This week they have said nothing, but I am feeling drop off and pick ups are a little awkward now. I'm not sure if it is just me perceiving something that isn't there, or if there is just a bit of tension between us now.
They came to me looking for temporary care with the possibility of staying permanently. I am at the point right now where I do not want to keep them permanently, but I am unsure how to go about telling them this. They said that their old provider got hurt and needed a few months rest, and that is why they needed care. I have only had them just over a month, but I'm already feeling a bit iffy with them, like something just isn't right.
So, how do I go about telling them that I don't want to keep them permanently? I don't want to ask them if they plan to stay now because I feel like that would be leading them to think that I want them to stay. I intend to give them until the end of August, as that should be sufficient time for their old provider to have rest or for them to find a new provider, if she is still unable to care for their son.
I thought about just writing a letter of termination, stating that things are just not working out and that I feel their son would be better suited at another daycare, but I would give them until the end of August to find alternate care.