I'm curious as to what you're planning for the meal service . . . all adults and kids (20ish people) sitting at the same properly set table? With the food items being passed around? Or were you thinking buffet-style?
Without knowing your service plans, I'd like to suggest that a special table be set just for the kids where food is served to them in small kid-friendly portions (kinda-like mommy being the "service maid
" for the princess and her guests) and then the adults can mingle with a buffet style service.
I dunno. Maybe I'm over thinking.
It just sounds like the kids (especially your 3-yr. old) might get a bit lost with things like "Condiments for sandwiches - mayo, yellow and dijon mustards, leaf lettuce, onion, tomato sliced thin." or "Hot Spinach/Artichoke Dip? Baguettes" Which is perfectly fine for the adults.
Maybe having a tray of pre-made child-variety finger sandwiches, along with fresh veggies and ranch dip, fruit (wands? Great idea!) would make the tea party a little more special for the dressed up and proper kids at their own table (plus, probably, make things a bit easier on the adults.)
Just my thoughts.