how about a bouquet of 16 purple roses?
I have another idea for you, lol. (My nieces are 3 and 4 so we watch this movie A LOT.)
For party favors, you could buy some colorful bobby-pins or snap clips and glue/sew colorful buttons to them.
What about 'pin the button eyes on the other mother'?
Or hanging streamers in a spiderweb pattern?
I know you have the cake planned out, but this is really cute as-well and seems fairly easy!
Good luck! (: