We have a chore system for our two tweenagers, where we all clean up at the end of the day and they help out. So far it's been pretty good with less and less oversight needed over time, and relatively few arguments.
Recently I found out that when taking out rubbish, one or both of them have been just slinging it over the fence rather than putting it in the compost heap or the bin (small bags not big black bags). This doesn't seem to be every time, but it was at least 6 separate times from the amount I've just cleaned up, inside the last few weeks. Two times it was a nappy bag from our youngest which is particularly irksome.
The odd thing is, they actually walked out to where the compost heap is, then rather than lifting the lid and putting the bag in, just threw it from there. The bin for non-compost waste is in the opposite direction, about the same distance - so throwing nappy bags that way seems malicious. One was up a tree.
The trouble is, I don't know how to find out who did it and I'm not sure on an appropriate response. I'm pretty angry that they've fouled the nest, so to speak, and that they've acted so irresponsibly or maliciously.
Some background
One is sometimes prone to dishonest and sneaky behaviour, 12-year-old boy. The other is prone to burying her head in the sand and coming up with terrible solutions to problems (e.g. hiding rotten food rather than throwing it out until bad smells cause issues) and has a defiant streak a mile wide, 13-year-old girl.
I'd like to nip this in the bud, with a big focus on the responsibility to deal with rubbish appropriately and keep our house and neighbourhood clean. I want to raise the kind of kids that would pick up rubbish rather than drop it.
Question
How should I discipline them (both, or try to find out who?), making my anger and disappointment clear without garnering resentment or further littering?