Oh man, I hate it when other kids come over with goop on their fingers and proceed to wipe them on the walls or carpets. Since you're not a parent, I'm sure its an even worse feeling :-)
The best defense is usually a good offense. Meaning, as the family begins to walk into the house, thank them for taking off their shoes. Maybe leave a towel out on the floor with a pair of shoes on it where they know they can leave theirs. Firmly insist.
When the kids eat, politely remind them that the table is where they eat. Buy some wipes and make sure they wipe down their hands when they're done. Or buy a stool so they can reach the kitchen sink to wash up.
When they make for something that you don't want touched, re-direct them to something that is OK. "The shelves have lots of interesting things I know, but there are some pretty neat things in this cabinet I've set aside just for you." (And having a few toys, crayons and things in a credenza for them would be a nice touch)
Its not OK to discipline other kids, ever. It is very acceptable to establish rules of conduct in your own home. You might be a little cleaner than most parents, but that's your privilege. However, beware that the parents won't like that you've stood up and announced that your house is your home and that you want it treated with respect. And to keep the peace, some things might be better noted and cleaned up after the kids have left...they are kids after all :-) GL!!!