Make them watch a lot of Popeye. A LOT.
Serve it in nugget form.
Tell them that eating spinach makes you turn into The Hulk.
Serve it in dinosaur-shaped form.
Tell them Santa changed his list from “Naughty or Nice” to “Ate Spinach or Did Not Eat Spinach.”
Barring that, bribe them with candy canes.
Tell them it's a special kind of cereal they've never heard of.
…and they have to reach the bottom of the bowl to get the toy.
Wrap it in bacon.