Honestly, if I saw a child at a scientific conference, and they were sitting quietly and paying attention, I'd think "great - a budding scientist, getting exposure when they're young." There's no specific age at which science goes from "blahblahblah" to "Wow, what a brilliant idea!"
Now, if they're running up and down the aisles, that's a different thing.
Well, it could just be a matter of opinion. But, most of the kids I see are very young, like four years old or less, sometimes even toddlers/babies in strollers. But even the ones who are a bit older, I mean I suppose they could be extremely bright and motivated kids, but most of the presentations are very technical, often delivered by people who aren't particularly dynamic speakers, in rooms with poor acoustics. They can be hard to follow even for people in that field, with years of experience and advanced degrees. I wouldn't think it so odd if the parents brought the kids to see one of the plenary sessions where the talk is by a famous scientist the parent admires, but 95% of the sessions are just regular, workaday things, and always geared towards others in their field, never general audiences or even people in other scientific fields at the conference, let alone kids.
But I have to admit that I, personally, often find the seminars dull, so I might be biased into thinking there's no way a kid could enjoy them. But I think there are a lot better ways to expose kids to science than by taking them someplace that is basically meant as the pinnacle of professionalism and networking--a science museum, a hands-on demo at school, even a well-done TV show like Cosmos.
Well, David Stuart, the Maya scholar, gave (not attended) his first presentation at a major conference at age 12.
I agree 4 year olds would probably find a physics lecture not to their taste. If the number of children taking up chairs is having a significant impact, I think the conference needs to address it. A well-mannered, quiet child at a presentation doesn't strike me as outrageously unprofessional, if they are not otherwise causing a distraction.
Oh, I don't know - my father did once find me watching an Open University program on the subject of Quantum Mechanics with rapt attention when I was about 3...
On the family-at-work thing: The first morning I drove to work on my own, my father turned up at our office mid-morning. Ostensibly, he'd had something to deliver in the area and was "just passing", but the reality was he wanted to make sure that I'd made it safely to work without any problems. It was simultaneously very sweet and highly embarrassing. Fortunately, I have a very understanding boss!