Question

The child comes back from the fathers-and-sons program with half a bag of Bamba, the first half already in his belly.
The little child cries out that he also needs Bamba, and there isn’t any at home.
The father suggests to the older child that he give up the half bag to the younger one, and on Sunday he will buy him a brand-new full bag at the grocery store.
The child agrees = to invest half a bag on the Sabbath in order to receive a full bag on Sunday.
Someone wrote that this is interest.
It doesn’t seem so to me, because this isn’t borrower-to-lender, but a third party, so it is rabbinic.
The Bamba was given to the child, but it wasn’t said, and there was no explicit indication from the organizer of the fathers-and-sons program that the father has no rights in it, so it is the father’s property, and the father can’t borrow from one of his own pockets and repay to another pocket the next day with interest.
Even if we say that the father has no rights in the Bamba, the child still cannot transfer ownership / lend to the father, just as he cannot return the etrog to his father’s ownership, so the child didn’t lend it in the first place; in any case there is no transaction here at all, and no deferment of money, and nothing.
At most, this is the child’s agreement not to scream while his younger brother eats the Bamba, and the father bought the silence in exchange for a full bag of Bamba on Sunday.
So what we have here is normal household management, not the “laws” and “legalities” of interest, since they’re all minors supported at their father’s table.
 
Am I right?