Would you be concerned if your inlaws adamently said they'd babysit one of your kids but not the other?

Mother in law asked us to participate in an event that would let her off the hook, and in turn offered to watch the kids. We agreed then showed up at their doorstep where father in law says adamently that he is willing to take one child...not the other. I was in the car and it was loud enough for me to hear him say this to my spouse while at the doorstep and I in car.