Our oldest has severe food allergies. Our younger two take it for granted.
When the birth order is flipped, it's a little harder. The younger ones
take for granted what the older ones have/do.

You might start by finding special foods (e.g., dairy-free ice cream) that
are special enough to be replacements. Explain to all that it's a special
replacement for the foods that the allergic one can't eat. You could even
tell the non-allergic ones that they can't eat it, to sort of even out the
imbalance. It's a tough problem, and you'll have to be creative, but firm
in your resolve. Explain over and over that this is just how it has to be.
And show the deprived child just how much you really care by taking extra
steps (even together) to seek out special recipes, special foods, etc.

Good luck

Mark
______________
Mark Feblowitz