REO Speedwagon's Kevin Cronin -- Heard It From My Son ... Bleeps Her Right in the Bleep

Kevin Cronin -- lead singer of '80s rock group REO Speedwagon -- heard the words straight from his own son's mouth ... and still couldn't believe it -- but it is one of the funniest celeb kid moments ever.

Cronin was shopping with his wife and kid when he and our photog started riffing about the lyrics from his hit, "Take It On The Run" -- and that's when his son pounced.

The kid upstaged dad by dropping the six-words that have been haunting live TV reporters for a while now -- but his tops 'em all ... mostly because of Kevin's jaw-dropping reaction.

You gotta see it -- he's probably grounded for life, but the total shock and awe on Dad's face might be worth it.