Yes, this arrangement is quite a hassle for you and a disturbance for the little ones. I know you need the income, but I would let her know now so she can start looking. She might wonder why you waited till January if it is a big hardship for you, especially when you did volunteer to do the care. You could give her a months notice or bite the bullet and say till the end of the year. Either way, let her know she must have alternate care at the agreed deadline.
As for wording, I would just say it is a hardship for the daycare to do the pick up. The children are negatively effected and the trip is an inconvience to your daycare schedule.