Here, that person is always a lady whose job involves taking care of the kids, for example, when they need to go to the bathroom. She may be assigned other duties as well, such as keeping the preschool's bathrooms clean. I know words like babysitter, childminder, nurse and nanny, but I don't know if any of them would work for this specific situation.
In the US, at least, the general class of professionals who take care of children are termed Child Care Workers. Within this group there are many different titles, depending on the job: aide, attendant, assistant, helper, coordinator, staff, or just worker, often specialized to different levels of care, such as daycare, baby care, preschool, and so on.
Here at Children's Academy, in addition to our teachers we have a number of daycare staff to help look after the kids.