New baby for d/c parent but daycare currently full. What to do?

[Naftafia]

I am having a really hard time trying to figure out what I should do and how. I am really hoping to hear all your thoughts and experience with this!!!
Here's the story!!! My daycare is currently full with 2 full timers, 2 before and after school and one currently part time (the mom who is on mat leave). So the plan has always been that I would take on the baby in early February and the sibbling would return full time, however this means I will have to let someone go.
I am having such a hard time with this because all my kids and families are great and they have all been with me for over 2.5 years. The only option would be to let my 6 year old go but how? I feel like I will be letting them down. I am having this ongoing internal/emotional debate as I know it is a business and the right thing is to take on the baby, or I would also loose the sibbling. Even worse is that I know the dad (single parent to the 6 year old in question) will understand... but it just does not feel right.
All thoughts are welcome!
To all who have done b/a school care I would also like to know how you typically proceed because if everyone stays you will end up with only b/a school?