Invitations - "Spit Spot! Make your way to No. 17 Cherry Tree Lane to (birthday child's name here)'s Mary Poppins Practically Perfect Tea Party!"
You could use Mary Poppins clip art or even make them in a teapot shape.
Menu - Tea sandwiches (PB&J, ham&cheese, cucumber, etc.)
Fruit Platter with dip (mix one 6oz container flavored yogurt with 8oz container of thawed coolwhip, refrigerate and serve)
Supercalifragilisticexpealidocious Tea Cookies (oreos dipped half in white chocolate with sprinkles)
Teapot shaped cake or a round cake with carousel horses on top.
Tea, of course! If the birthday child is not fond of tea, substitute lemonade, chocolate milk, or another favorite.
Depending on how nice your set is, it would look lovely to have a real tea set for the girls to use.
Activities - Bert's Chalk Drawings: tape long rolls of paper to the floor with chalks available for the children to draw with.
Let's Go Fly a Kite: Decorate butcher paper kites
http://www.allfreecrafts.com/kids/paper-kites.shtml
Get measured by Mary: tell the children that Mary Poppins sent her magical measuring tape for the party. In advance, prepare an appropriate saying for each child (such as Mary uses in the movie), and measure each child. Naturally, the birthday girl gets to be "Practicall Perfect in Every Way"!
Decorate a Mary hat: buy inexpensive straw hats at a craft store and spray paint them black. Let children decorate with red cherries and daisies. Glue takes a while to set, so with good adult supervision hot glue may be a better alternative that also allows the guests to wear the hats almost immediately and is great for photos.
Decorations - Lots of daisies! Parasols, and don't forget a sign on the front door that says "No. 17 Cherry Tree Lane"! Colored kites hanging from ceiling.
Favors - A spoonful of sugar of course! Dip plastic spoon in melted chocolate, and then sprinkle chunky colored sugar while still warm. Let cool and set completely, then wrap in cellophane and tie with curling ribbon.