ooh, my DD is just like yours! She is 3 next Thursay and her party is Friday and I was also ordered to do a princess party (with pirates for the boys)
DD learnt all about princesses at Disneyworld and DLP. We have bought her a sleeping beauty dress for her birthday with tiara, wand etc and assume she will wear that.
With regards to the cake, I must have the same cake book as Takver as that is exactly what I am doing. My mum bought me a fab childrens cake book years ago and every year I've made the cakes for my DC's parties. DD1 (now 13) also had the princess one when she was around 5 so I'm hoping a second attempt will be better.
I'm making a square (pink) sponge cake, cut the corners off, around 4 to 5" high slices of swiss roll on each corner and one in the middle with an ice cream cone stuck (with butter icing) on top of each one. Cover with loads of pink and white butter icing (I cheat and use betty crocker frosting and bung some red food dye in for the pink) wafers for windows, little sugar icing flowers on the front and use icing pens to do the stems of flowers so it looks like they are growing up the front. Loads of sprinklies all over the place and if you feel up to it, put it on a large board with green butter icing around the 'castle' and then some blue butter icing in front of the door (made of wafers) to look like a moat. Its amazing what you can do yourself and they are three, they won't notice if it doesn't look like in the book! It will look cute and DD will love it.
I'm a bit stuck on games as 3 is a difficult age as most will have the attention span of a gnat but am going to do a few games. Am thinking of 'stick the tiara on the princess' instead of tail on donkey and musical statues or similar.
We've got a bouncy castle for the bigger ones (ie 3 and 4) and bought a small little bouncy castle for the little ones (1 to 2 years)
face painting, music, balloons, will be fab.
I must admit I gave in and bought all the Disney princess stuff, plates, napkins etc and party bags and bought some brilliant (and cheap) tiaras in sainsburys that came in packs of 4 or 5 to go in the party bags with some sweets. Disney princess colouring sets as prizes for games.
If you haven't got a 'proper' princess dress then any pretty one will do and if you haven't got a tiara you could make one or buy one of the sainsburys ones (or similar)
I'm sure she will have a lovely time