I agree you did the right thing. That particular exercise was terrible because what it did was to teach the kids that God offers handouts when requested, like asking Santa for Christmas presents. God doesn't work that way. He does answer prayer, but not necessarily in the way we had in mind, and He provides what we need, not simply what we want.
Those kids learned a bad lesson, but your honesty mitigated some of that.
"He who takes offense when no offense is intended is a fool, and he who takes offense when offense is intended is a greater fool."
"Don't take refuge in the false security of consensus."