This is difficult, and I learned the hard way. The BEST is to start when they are VERY YOUNG (like 2 years old). Otherwise my wife cooked chicken in many different ways (for protein) so the kids now know which kind they like/dislike. At first they turned their noses away, but with treats and common-sense, we bribed them to finish their plates. Whether it’s dipping it in ketchup(their favorite way to eat chicken) or to give them delicious drinks, etc. Eventually they enjoy and like it, they now eat a whole lot of protein. BTW, when kids eat protein, they are MUCH EASIER to try new foods. Kids who eat no protein, are fussy eaters and have a hard time eating new foods!
Only Shabbos do we serve sugary cereals. Weekdays we have a variety of healthy cereals from which they can choose from-and they love it! For example, today we had corn flakes with sliced pieces of bananas for fun! Fish is not a good seller, unfortunately , and my older kids won’t go near it except gefilte fish. My little ones eat fish, b”h.
hatzlochah!