I had a situation where I had all 2-3 year olds and then suddenly had a 1 year old join us. It had been my 2, 3 year olds for over a year and it was getting pretty easy. We had hit our stride.
When I introduced the baby I also wondered what the heck I was thinking and whether I had made a mistake. The baby (of course) didn't want to play with any of the other kids and needed so much more attention and care compared to the other independent toddlers I had.

The 1 year old really was just not able to do much of anything...couldn't walk, couldn't self feed but I knew that if I hung in there that around 15-18 months it would change.

Now it has been 5 months and the child is 16 months and I am loving it. The 1 year old is interacting with everyone and all the other kids love the baby and she is walking, talking etc and much more interesting to everyone now. Hang in there when the baby gets to 15 months approx and you will see that they will become a bigger part of your daycare.

Good luck