Presumption & Advantage Taking

Yesterday, when I collected my 4 yr old son, Ed from school, the first thing he said to me was;
"Where's Jack's Mummy?"
I knew exactly what he was after.
Jack's Mummy has chocolate coins in her pockets for her kids when they get out of school (a practice I'm not sure how I feel about & will be discussed very soon).
Jack & Ed are good friends. They play together at school everyday, but with this friendship comes bickering and the occasional fisty cuff fight (in a 4 yr old-nice-boy-really way). They often end up in pointless fights over silly things, which we always work through. Thankfully (since I'm so new to this mummy vs school thing), Jack's Mummy is wonderfully approachable. In the beginning when Ed came home telling me Jack was hitting him, I could go to her and we solved the miscommunication the boys were having with no problems.
However, because of this occasional bickering, poor Jack's Mummy does seem to feel a bit guilty. She often shares her children's chocolate coins with mine, which is fine by me, I can often use it to my advantage & bribe them to get to the car/walk home nicely.
But, for Ed to presume that she will give him a coin everyday is definitely a step toooo far.
I know it's very easy to take advantage of other people, it's really easy to slip into presuming someone will help/provide/etc. I often have to berate myself for it. A lot of different people have helped me out in a lot of different ways, & I endeavour to remain grateful & courteous, forever. I do find it very difficult, because I seem to be by nature a particularly selfish and inward focused person, but I try really hard to never forget.
Just because someone has helped me out once in a while, doesn't mean that they will/should/can ever again. 
And I do not want to raise two boys who think they can presume to expect anything from another human being. I want them to be independant, but to be able to accept help graciously. Having friends you can fall on in times of need and whom you can be there for when they need you is one of the joys of life. I guess it's about finding a good balance.

Ed never got his chocolate yesterday. I had told him he could have one out of my secret stash (because bribery works) if he walked to the car nicely, but then he ran into the road without holding my hand & therefore lost it. Silly boy.

Any thoughts?