I have a rule, if a child cant have something because of dietry requirements I cater to that, anything else they get what they are given (within reason, have one child that really does not like beans, so we dont have them when he is here for lunch)
I have one 5 year old that wouldnt eat anything but fish fingers and chips, or cocopops, wouldnt even try anything else when he started with me in July of last year.
I started a pasta jar for him, he put a piece of pasta in for everything he tried, he wasnt offered anything else until the next snack/meal then.
It took about a week of being really tough, but he started to try things (and by try, they have to ahve a proper teaspoon, not just lick things which is what he would do, one lick and say yuk, which was rubbing off on other children)
Anyway the break though was custard, he loves it, and said to me, 'why didnt I like custard!' I explained to him it was because he hadn't even tried it, so he didnt know he liked it!)
He is now sat in front of me eating scrambled eggs on toast! (was on a overnighter last night)!