Help! When to stop lifting

I am totally confused. I work at a child care center and am not sure when I should stop lifting. Most of my children are potty trained but I have one who wants nothing to do with it and the child is extremely heavy. I have noticed on the days the child is absent I feel fantastic but when they are there I am in so much pain halfway through the day. Is there a set mark when you should stop lifting  and is there a certain weight limit?