Never let your kids open the front door
This guy worked as a door-to-door salesman for all kinds of products. He wasn’t a burglar, but if he were, it wouldn’t be very hard to burglarize the homes he visited. The OP was shocked to see just how many parents allowed their young kids to open the front door and greet total strangers.
Burglary would probably be the least of your worries if a stranger just snatched your kid away because of your negligence. We get that you might get distracted by whatever you’re doing, but don’t teach your kids to open the door to strangers. Teach them to call you when someone comes knocking.