Parenting?

Ok... this one is a bit more of a questions and purge than anythign else.

I realised today that I expect a lot out of my 7 year old, especially since he has been having a few problems with behaviour at school.
Instead of reducing my ideals on his behaviour I seem to be expecting more of him.... Is this normal?

We were out with two other kids today Miss8 and Miss5, sisters. Now Sisters fight and poke and jeer each other, but my LM seemed to want to get involved. Not only in the poking but also in the parenting.... stepping in and telling them to behave.. which I promptly told him not to.

I also found myself holding the other kids by the same rule even thought their mum was with us too. I guess I don't think it' fair if they are able to do things that he isn't ... but at the same time is it really up to me to do that?

I also smacked him in front of the other kids on the beach, unfortunately he didn't have  a shirt on which meant it stung a little more than I meant it too, but he also did the exact thing I had told him not to only minutes before.  Should I have not done that in front of the others, would this have embarrassed him? he didn't cry, though I know it really did hurt but he just walked it off over to the water to wash off the sand. By the time he was back he had sorted himself out.

How do you cope when you are looking after other kids? or when you have other kids with you as well as the parents? What ideals do you hold a single child of 7 up to? what should I expect of him and what is too much?