Do you have a gate to your backyard? If you do, lock the front door with a note that you are in the back and let them come through the gate.

In the summer we're almost always in the backyard in the afternoon, so my parents usually just come through the gate to get their kids. I have all the bags out back on the deck. This prevents people from wandering through my house and prevents me from having to take one kid out to the front door while the others are in the back unattended.

I am appalled by the thought that there are people who wouldn't automatically take off their winter footwear to walk through someone's home!