Would you keep her on if those behaviours improved? Sometimes parents need a swift kick you-know-where lol... It is very difficult for a child that age when part time. I started an 11mo full time in November, that was on bottles/purees and firstborn and by Christmas she was improving (would eat some food, still screamed if I left the room) but now this month she started walkig and is super independent, eating lots and even trying cutlery. I was straight up with the Mom in the beginning, and never fed purees (unless yogurt, etc was on my menu), just gave soft finger foods. She's still on formula bottles, but I've gotten her to drink water from a cup at the table now, so those will be next to go.

What if you wrote a letter explaining to the parents that you feel she isn't settling in to group care? Perhaps write " Dear X, DCG has not yet settled into our group. [detail a couple of the things that are an issue] I would like to make a plan together to help her complete the transition period. If no progress is made over the next 2 weeks, I regret to inform you that based on her current needs, I will need to terminate care effective March xth."

This way your month notice starts now, but gives them a second chance. If you are just done, then remove the line about making a plan and the next 2 weeks part.