I adore afternoon tea. One of my favorite books written on the subject is The Afternoon Tea Book by Michael Smith. It was written in the 80s, but it is a great resource. The first half of the book is a history of all things tea and the second half is filled with recipes. It really is an amazing book and worth tracking down.
For menu planning help, check the afternoon tea menus at hotels & tea houses. Here are a few to get you started:
The Ritz London
The Plaza, NYC
Alice's Tea Cup
The Grand Floridian, Disney World
American Girl Afternoon Tea
With the ages of your children, however, the key is to keep it simple but cute. You can put together a beautiful tea with simple tea sandwiches (peanut butter and jelly sandwiches, ham and cheese/turkey and cheese, cucumber sandwiches...), scones and cream, and sweets. The sandwiches can be made special by just using cookie cutters to cut them out. For sweets, you could make mini cupcakes, shortbread cookies, chocolate covered strawberries or chocolate covered marshmallows dipped is sparkly sugars, fruit cabobs, fairy bread....I could go on and on (can you tell I love afternoon tea!) I guess it is really all about presentation...make it small and cute :)
For the tea itself, you could do a traditional black (Twining's Afternoon Tea is a favorite of mine) or an herbal tea (my 4 year old love's Stash's Wild Raspberry Hibiscus). Pink lemonade, hot chocolate, or apple cider also work really well.
I've went on too long...I hope this helps out a bit. Have fun with your tea parties!