You are killing birds
Montgomery County is famous for being a fairly conscious-of-the-environment sort of place. It's the sort of place where when I first met my local county councilmember he had biked to my house, and where you run into some elected officials at the healthy food coop, and even more at the Whole Paycheck Healthy Food store. So, unlike when I was a child, we try to justify not wasting so much not with an appeal to "hungry people in China" but with "God wants us to save the beautiful things that have evolved in this lovely planet." Sometimes the phrases aren't phrased so well.
First of all, environmental consequences are fairly complex chains of cause and effect and trade-offs. We have run-off, toxicity in plastics, human health effects, nutrients in the spawning waters of imperiled oceanic fish populations, deforestation, and so on. So while being as aware as one's own scientific ability allows, one has to simplify things a bit for the younger audience.
"If you waste the toilet paper, they will have to cut down more trees, and the birds will die."
Never mind, that I buy toilet paper out of recycled paper, it seems to me that if we play fun but silly games with the toilet paper, some how that's putting upward pressure on the price of lumber, and more clear cutting will happen, and more diverse forests will be destroyed for tree mono-crops. (Note for the horrified parents out there, this phrase wasn't spoken as part of a toilet related action, but as a part of a much more frivolous game in which toilet paper was being wasted; my standard approach on excessive toilet paper usage for toileting purposes is "use as much as you want to feel comfortable, just flush every so often." Not so environmental, but better than stopped up toilets, and I figure that when people have the autonomy to solve problems, the solutions end up being not so wasteful as an imposed efficiency.)
I never did really find a better way to explain why not to waste toilet paper.