This is a very hard topic to deal with. If they are determined to take a photo of your child, they will all but stalk you to get the right shot. My daughter (6.5) in particular has been "paparazzied" (love that term Nicolejoy) since the day we got to HK. I go through phases of 'eh - whatever' to 'extremely annoyed' and so does my daughter. There are a few hot spots for us where it can be almost prolific - HK Disneyland, Ocean Park, anywhere in TST and the beaches being the worst. It is mostly mainlanders, but we've also had issues with Japanese and South African tourists among others. We had several incidences yesterday at HK Disneyland where people were constantly trying to get a shot of my daughter to the point I just put my hand over her face for much of the time we were there. On top of that, we had a mainland chinese man keep trying to get close enough while we were in line for "Small World" to try and touch her. He reached out a couple of times and I pulled her away so that she was eventually shielded by my husband and myself. It is one thing to take a photo, entirely another to touch. Some people seem almost obsessed with wanting to touch her hair and head. What is up with that? She hates it and it creeps me out.
Most of the time I ask my daughter if she is ok with people taking her photo and depending on her reaction allow, or not. Of course, people don't seem to understand no and just become more discreet (or so obviously indiscreet you can't help but roll your eyes) about it.
PurpleFlower - what that person did is totally NOT ok in my opinion. If anyone tried to wander off with one of my kids, plane or not, I would be going off at them and complaining.