Does it bother anyone else...
okay, so Dennis is in the garage getting paint, sees his sling shot, drops the paint, and begins vacuuming the paint up.
Polly shows up to babysit and the parents just leave. They, obviously, didn't go tell Dennis goodbye otherwise they would have seen what he was up to and therefore would have avoided Mr. Wilson eating "paint and wood".
I know it's just a movie but that part always bothers me. I am a mother of a one year old and I never ever leave the house without telling my son bye and giving him a kiss!