That kid is learning all he needs to do to get his way is throw a fit.

Or he's five, he has a lot of big emotions, many of them centered around food, and being expected to eat something unfamiliar on the fly when everybody else gets to eat something they enjoy is enough to send him into a meltdown, which LW calls "throwing a fit" because they don't have better vocabulary or understanding.

Could really go either way.

I also think that the LW just didn't really want to make the cheesecake.

Yeah, I got a hint of that too, in which case LW's weaponizing one child's problems to use against the other, which is a level of parenting so bad it falls off the scale.

Though really, it's not that hard to buy cheesecake at the bakery.