Regardless of what you serve, forcing children to "clean their plate" even when they aren't in control of the portion size is very common and leads to strangely adversarial relationships with food as well as skewed ideas of fair portion sizes themselves.

Much better to allow them to serve themselves and permit to go back for more, while still maintaining a "eat what you take" rule, than force them to choke down an amount of food they didn't choose until they're feeling physically unwell.

If they don't eat enough, they will reach a steady state eventually. If they over-choose, they will regret that and correct in future. Some level of veto is necessary to prevent gaming of the system, but there's a difference between "you can't choose a single spoon of rice for your main course and fill up on ice cream later" and "eat this half-kilo of pasta because I said so and you'll sit here until you can miserably force it down".