I wouldn't sweeten the tea, I'd let the kids do it themselves with little bowls of sugar and spoons, that's the fun part :) Or even better, get them sugar cubes to drop in (then you can control the amount of sugar they add a little more easily!)
Sounds super cute, I love stuff like this. Other sandwich ideas are egg salad, chicken salad, or just plain turkey and mayo on white bread. To be extra special you can use cookie cutters to cut the bread into fun shapes.
Get the decaf tea, usually around this time of year they have gingerbread tea and peppermint tea in the stores, celestial seasonings is a good brand. My daughter like the process of a tea party but not so much the actual tea-drinking part. We add milk and sugar and it's more like a chai latte, mmmmmm.