Easy as that.
The degree of change is a little too weighty to be going along with. I think I think at the margin all the time, yet I’m not exactly sure I could concisely articulate to myself and others what I’m actually doing.
Consider this a personal favor. I will award this beautiful handpainted asterisk, *, to the person with the best explanation.
Pretend you have a list of things to do but you only have so much time to do them. Let’s say there are 10 things on the list, in order of their importance. If you only have time to do 7 of those things, then we call #7 and #8 marginal values.
If for some reason, you got more time or less time available, and you could do more or less things, then that’s called a marginal change.
Marginalism is the idea that in order to see how a person should make a certain decision, he should look at how much important those marginal values are. If they’re very important, he should try to make them happen. If they’re not that important, then his resources are better used working on a different list, like a shopping list.
I’m not really sure what you mean by marginalism.
As in economic theory? That seems a bit steep for a 5 YO.
Or is there another concept you are referring to that I’m unaware of?
Perhaps you could talk about the things we would do no matter what: sleeping, eating, going to the bathroom. Then go on to the things we really like: certain TV programs, playing with mom and dad. Then go on to activities that are ok, but not a big deal: a particular toy, getting a hair cut. As you move out from the core (eating, sleeping, TV?) you get to the edge, the margin, the place where you can take or leave it.
Anyway, that’s what I’d do if I felt compelled to explain it. If I even get it myself.
5 Year olds are very concrete learners. Help her to define it for herself, by getting her to make a list relating to her own life. I’d choose one group of tasks s/he’s very familiar with that happens in a routine:
Ask her to list all the things she does to get ready for bed.
Brush your teeth
Wash your face
Put on your pajamas
Read a book
Turn on your alarm clock
Set out your clothes for school tomorrow
Find your favorite stuffed animal
Tuck in (or get tucked in)
Kiss your loved ones good night
Put on your music CD
etc.
There are some things on this list that are absolutely positively important and **must **happen (they may vary by household, but in ours, they’d include teethbrushing, finding Lovely the stuffed dog, tuck in and kisses). Ask her to identify those. Those are definitely NOT marginal!
There are some things which are nice to have, and if they don’t happen there may be a few tears, but she’ll still get to sleep eventually (in our house: pajamas, read a book, music). Ask her to identify those. These are marginal, but marginally so. 
Still, they’re important to include so she doesn’t equate “marginal” with “stuff I don’t like”, which is the next category:
There are some things which get skipped quite often and indeed, she may even prefer it when they’re skipped! (Alarm clock and clothes for tomorrow don’t happen on weekends, for example. Facewashing gets skipped whenever Mom is distracted and doesn’t remind her!) Ask her to identify those. These are the truly marginal ones, and easy to identify as such.
It’s that middle ground that’s tough. What may be marginal on that list for one kid is vehemently not for another. And that’s true in economics, as well - deciding what exactly is marginal is fodder for many a Congressional Budget Committee and School Board meeting!
Damned straight…I barely understand the concept myself.
I wasn’t exclusively thinking economics, but just marginal concepts in general. I think marginal utility/product of labor/cost/benefit have some common theme that can be dumbed down for a child, whatever the hell that theme may be.
My understanding is that marginalism eschews the overall for the here and now. How would a five year old marginalist make his career goals of bounty hunter tractable? Learning how to write sentences is more productive than running away to be a crime fighter. Even though he might eventually catch a bad guy, he has more teachers at his disposal than guns. Then, when he’s on multiplication, he should reevaluate his resources/endowments.
Is that still marginalism?
Jesus, what the hell kind of little monster are you trying to create?
Play a game. Take a big handful(for an adult) of marbles. Tell them that each time you give them a marble they have to jump up and down and cheer. They also have to hold all the marbles you give them in one hand.
The first marble, big cheer, lots of jumps. Maybe the second through fifth or so too. By about the twelfth marble, the cheers are phoned in and the jumps are little hops. Eventually the cheers will die down. By the time the marbles are overflowing their ability to hold them in one hand, they’ll start getting annoyed with the game. More of a good thing rapidly becomes blah, and too much of a good thing is annoying.
Adults have a word for being less excited about the twelfth marble than the first. It’s Marginalism. It applies to too much video gaming, too much food, too many clothes, etc.
Enjoy,
Steven