When do your start teaching a child to wipe their bum?

I have a 4 year who has been potty trained for about 2 years now. He's going into preschool this coming fall and I feel like he should know how to wipe his own butt by the time he's there. I'm wondering if anyone has found an easy way to teach their little one how to wipe their butt without making a mess. I'm finding it a little difficult to do. He willingly tries and doesn't do too bad, I'm just wondering if there is an easier way that I just don't know about. via /r/Parenting https://ift.tt/38HioW8