Potty training age

I run an in home daycare. I have a rule in my contract that by three years old children need to be potty trained or parents making a honest effort. I have also told all my parents I am more than willing to help with the training as long as they are also at home.

I have had my in home daycare for a year. I have worked in two different daycare centers, been a nanny for multiple families, and babysat in general in the past.

Do you guys think this is in unrealistic expectation? I know many daycare centers and preschool require them to be potty trained by this age. I have a parent who has a three and half year old and is upset by this requirement as they haven’t even attempted to potty train.