Oh Man! been where you are tooooooo many times, agree with other responses of..."it's not your job"...IF the parents were willing to do it at home ALL THE TIME. that would be one thing, but the fact that they are clearly "lazy" and not doing it but on the weekends? Dont waste your time anymore....I had a boy who was nearing 4 and still not trained, got very frustrated but instead of tryingto do it on my own, I communicated to the parents that if he turned 4 and was not trained I could no longer care for him. He was in underwear before he turned 4. Some people really are quite lazy for what ever reason, not enough time, just don't care, or "I asked him and he said he didn't have to go" stupid. (sorry but true)
bottom line..if the parents aren't on board, don't waste your time. Mixed messages will not train the child, only consistancy will.
If it bothers you enough to have to diaper a child you know knows better, you could add to your contracts that any child above the age of 3.5 (or whatever) must be potty trained. of course keeping in mind children with special needs or learning disabilities and such.