I once had the pleasure of sitting next to a 9 or 10 y.o. boy traveling alone. I wish I could have sent a note home with his parents thanking them for the great job they did teaching their son manners and consideration. He chatted pleasantly, but not much. He spilled a little clear soda on me when there was some turbulence, but not as much as my DH usually spills on me. At least the boy apologized, I accepted, and let him use the paper napkins I always carry with me to clean up. Plus - he was small so he didn't overflow his seat and expect (somewhat slender) me to surrender the armrest and half my seat.
I will not tolerate small children kicking the back of my seat. I ask them nicely to stop once. If they continue, I ask the adult with them to please control their child. If the adult says, "It's not a problem," (gr-r-r) the next time the kid kicks me, I recline my seat to the max. This usually puts my head in their space. I say, "We are sharing this seat: You use this side, I use the other side. Let's be considerate of each other." I say it politely, but my tone implies that I will keep my seat reclined to the max if the child continues to kick me.