Here are some resources I found online...
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Here is a pdf file used by a library for their Reading Program, including suitably named medieval treats for kids... very cute, and easy to do!
http://www.wla.lib.wi.us/wapl/conferences/2005/05Med_menu.pdf
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Another more "authentic" menu is below. Of course, most of these things are WAY too out there for a kid's party, but I found the list of "foods to avoid as they hadn't been discovered yet" interesting!
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The following is an example of a menu of a medieval feast. Each course included a soup, followed by a wide range of baked, roasted, and boiled dishes, and finally an elaborate 'sotelty', a lifelike (often edible) scene sculpted in colored marzipan or dough. The bounty of medieval feasts is legendary. One early historian noted that in 1398, King Richard II [presided over a feast]. A variety of choice morsels was set out to satisfy a trenchman's every whim ... gilded peacock and festooned boar's head were highlights of the menu."
Oystres en Grauey--oysters steamed in almond milk (15th c.)
Brede--bread flavored with ale (15th c.)
Chawettys--tarts filled with spicy pork or veal & dates (15th c)
Pigge Ffarced--stuffed roast suckling pig (15th c.)
Goos in Sawse Madame--goose in a sauce of grapes and garlic (14th c.)
Caboches in Potage--stewed Cabbage flavored with cinnamon and cloves (14th c.)
Crustade Lombarde--fruited custard in a pie (15th c.)
Hippocras--spicy mulled wine (14th c.)
Things to include/exclude in a meal:
There are some foods that should not be on a Medieval menu because they were discovered in the New World and European medievals didn't have them.
To include:
Pigeon/squab
Fennel
Leeks, shallots
Apples, Plums
Parsnips, turnips
To avoid:
Squash, incl. pumpkins
Potatoes
Tomatoes
Chocolate
Yams, sweet potatoes