This! Any new skill needs practice. Nobody ties down a baby til they can walk, or tapes their mouth shut til they can talk. I start the toileting process once they can walk (talk about it, change them in the bathroom, let them sit on the potty while I take out a fresh diaper). Once they have skills to pull down their pants I ask for trainers or pull ups (so that the child can change themselves, just with my help as needed). Going back to diapers sends the wrong message (it's ok to pee in your pants). I'd be firm with the parents that you don't want to disrupt their success in your home-so you will be continuing with your (successful) method.