Are you being a jerk? Kinda, yeah... what do you have to lose by letting her? It's for your family, not for you. Do you love your wife and son?

Lots of people get their pictures taken every day by the media or by tourists, without being asked permission. If you think you have it bad, imagine what it's like to live in a small impoverished village that's on a tourist circuit, having rich white foreigners snap intrusive pictures of you and your family without asking. At least your wife is asking for the good of your family as opposed to a souvenir.

But of course it's entirely up to you.