I really need some advice here as I am not sure what to do...

I currently have 2 siblings in my care which I have had since september.
When I made a deal with the parents they told me 4 days a week (for these clients as I had just gotten started I told them they only pay for the days children come) turns out the children really only come 3-2 days only on very rare occasion have they come 4 days of the week. I am a little frustrated with the situation to be honest because she usually doesn't tell me until the night before they wont come and days vary though out the week that they don't show up, I can't get a part time client to fill the spot as there are no set days they will or wont be here. I feel bad for the parents, very nice people but I am not happy with the lack of attendance (I do understand its partially my fault for setting it up this way but I was a beginner, wasn't sure and wanted to be nice. now for all my clients the system is different, I learned)
I have considered terminating them due to this but never did as I said because I feel bad for the parents, they have told me how much they like what I do with their children and are happy that they have me for a care provider many times.

Now I know a lady that is searching for care for her child starting soon and she has expressed how hard it has been to find someone to have her child and would like me to do it. I told her if I had room I would take the child but I just can't at the moment because I am at maximum numbers now. I would charge for days booked in advance for this new one.

So my dilemma is: do I terminate the others to get this new one? If the siblings come 3 days of the week I get all the money plus $20 that I would have to work all week with the other child to make. If the siblings don't come I lose money...

What would you do?