Best method that works until they are 7: make up an imaginary friend (like a fairy, gnome, etc) that visits you once a year and brings you toys IF you eat the being's favorite foods, which happen to coincide with what the child detests

Works. 100% guaranteed. They will eat everything with a smile

In my case, I use the Tio de Nadal, which makes for a fun time when he visits (Kids have to hit the Tio with a stick and sing songs, then he shits sweets and toys)