I just re-read this again... I didn't realize that the children putting you over are summer kids. Those of you giving this woman grief should be ASHAMED. Do you really expect a provider to hold spaces 9 months out of the year to provide you space for 3 months in the SUMMER?! And who will pay her bills when she's being a bleeding heart? ... TAKE the babies and give notice to the summer kids. Summer kids are NEVER a priority with me. I take them if I can and don't if I can't. We have a right to make a living..... HOWEVER, my other advice still stands.
The only problem I see with this is that you are counting on both those babies being with you. I've been at this 26 years and learned some painful lessons. Many mothers have every intentions of going back to work after the birth of their 2nd baby. But, several things come into play. Many won't pay for the time they are keeping the first child out at all or you can't agree on the # of days if they will agree to part-time for the first child. Even if all that is ironed out, they often go back to work for a few days and or a few weeks and then throw their hands up and quit. If they don't feel they are making enough to pay for 2 children in daycare or they feel like it's likely their last baby, they suddenly want to stay home with this baby.
If I were you, I'd wait until both babies have been born and then give notice to one of the families if necessary.
On top of these concerns, are financial ones for you. If you have been toying around with increases in your weekly rate, then give the parents a notice towards the end of the school year that rates will change at a certain time. Then if someone leaves because they just don't want to pay more, then you haven't let go of this other family.
I know you believe you have great relationships with them and I'm sure you do. But when push comes to shove, they will protect themselves and their own budget and you will not be a high priority with everyone all the time.
I do have to say that I understand your thinking. Unlike some of the other posters here, I know that you need to look out for your openings long term. A full-time infant will be with you for many years and a part-time child that will move onto school sooner than later, is not the highest priority. But as I said here, there is never any way to be sure how things will shake out.
When my mothers get pregnant, I always hope to keep them and make it work out. I frankly would go over on kids for a few months until someone else moved or lost a job, before I'd tell one of my moms that she can't bring her new baby. BUT, that is not something I have ever had to do. It always works out one way or the other on it's own.