Talk to them??? I borrowed a very long, heavy duty hose from my neighbor just last week to fill a pool. The sprayer nozzle was locked in place. After about 10 minutes of struggling, I worked out a deal. He cut the hose apart, I use and keep hose, I buy him new hose and sprayer to make it up. Done deal.
My advice, just talk to them... There's a high probability that they just don't know normal neighborly etiquette since they're young and naive. Maybe they'll pay for your blade, maybe not. Maybe they'll just say "i owe you" and make it up some other way (labor, trade, etc.) If they're jerks about it, lower the blade and ruin their lawn, or dump chemicals into the septic vent to screw with them.