Language and grammar evolves and as far as linguistic changes go, the use of one preposition over another is pretty benign. Prepositional choice is already essentially arbitrary and varies from region to region.
English is a moving target. The current generation forming its own vernacular doesn't make in any more or less correct than when the previous generation did it. It's not as if English has its own académie française, and the flexibility of English is one of its most charming properties.
Leaving aside the condescending quip about toddlers, I don't see how over-regularization of prepositions is a bad thing. They're already confusing enough as it is and I personally would prefer a language with more consistent rules than not.
EDIT: A quick addendum: I didn't actually know this was a mistake and I appreciate having learned it anyway.