It actually sounds like you have your excuse. Given that you need consistency for the fall you are letting all part time families go. They will see it as keeping only the original full time. You have already told two of them that they won't be able to stay probably anyways because of siblings coming so it won't be a shock to them. You might be surprised and they all take it well.

My only concern is if it is really going to matter. In the long run even if you have one child in care you are stuck to the house doing daycare. Another option that many consider is to take on only part time kids and to take those needing the same days so you only work Mon-Wed but have Thurs and Fri to volunteer with your child's school, spend a personal me day, go to lunch with friends, work another part time job whatever.