I don't think it's rude at all when a child peers over a booth to see my table. In fact, I have been known to smile and wave at them! And now that I have a child, people are always waving to her, coming to our table to say hi, talk to her, etc., so I would never have thought it is rude when she gives someone an unsolicited wave. Actually, I feel kinda bad for her when she waves to someone and they don't wave back. If a kid is being happy and friendly, but them looking at (or waving) to you bothers you, then I would honestly consider eating at an adult-only establishment.
Granted my tolerance threshold for kid behavior is very high (and always was, long before my daughter was born) so a child peering at me is 'cute' to me, not annoying, but I really don't know all that many people who find it so bad, since most people we sit next to initiate an interaction with my daughter. So like someone else said, it's unfair to ask parents to 'read your mind' about a kid behavior that you find annoying, as what annoys you may not annoy the next person. A child who is whining, crying, on the floor playing, screaming, etc. does not bother me at all-I don't even notice it. As a mom myself, I often look to see who the child is, see how old they are, something like that (preview of what I'm in store for one day!), but that's it, then I go back to my meal. My daughter is usually amused by a crying child, so if anything, I like it, cause she'll sit and stare at the cryer forever-keeps her entertained!
The only time I have ever been annoyed by children in a restaurant was on a Princess cruise over NYE when kids from the next table kept trying to steal my daughter's balloon and actually went through my mother's purse. I had never (and haven't since then) seen anyone ever act like that in a restaurant, and I really think the problem was that the family was part of a large reunion group over 100 people, and over 50 kids. No one was watching any kids and so they acted like it was recess, which was really more amazing than annoying, but I would have preferred to eat without that mob in the restaurant.
So I guess my point is, as a parent, I do not think it is rude if my 11 month old looks at you during your meal, so if it bothers you, the best solution is to tell me and I'll be sure it doesn't happen again. Yes, I'll think "how strange" but better I think that and you complete your meal in peace than to say nothing and have a miserable dinner. All of these problems could be solved by telling the parents the behavior bothers you and then continuing with your meal. Sure, some parents won't care, but I'm thinking most would try to help you.
Luckily for the sensitive DISers out there, I learned this before my children were old enough to be out of their highchairs and staring at people. They're 4 & 1 now.
