What would you have done?

little kids are always full of questions, I don't think that the other mother got offended by the question that your 3 year old asked I think it might have been the way you reacted. Instead of trying to dictrac your 3 year old, it would've been better if you had explained it to him once more. I'm almost sure that the other mom thought that you were trying to keep your son away from her baby, and I'm sure that it seem rude from her point of view.