I know that us here at FT love to immerse ourseleves in the letter of the "law" when it comes to UA (usually so we have a better chance of working around it) but I would be astonished if this became an issue. I used to sit in the exit row with my father all the time (ORD-HNL), when either my sister or my sister and my mother were with us. I don't think an FA ever even asked. You would have to have the single most vigilant FA ever IMHO.

FT gods feel free to strike me down with someone responding to this thread with a "the time I lost my exit row seat because I left my wife and son in the row behind me" but I just can't see this given that the last few times I've been in an exit row the "briefing" lasted about 7 seconds "yes, yes, yes".