Firstly, to most children of that age you won't be a ventriloquist. To a child the doll will probably seem as alive and real as you are. Think back to when you were a child, if you watched a puppet on TV you weren't aware that it was a puppet, it was just another character, the definition just wasn't there.
So my first advice is to stop thinking you are a ventriloquist, think more along the lines of it being you and your friend visiting other friends at their party. The more you believe in your 'friend' the more everyone else will believe as well.
Then I'd have a look at some stories you can tell with the help of your friend to all your other friends, it can be anything you fancy, from the traditional fairy tale to something you've written yourself. Use this to build your structure, maybe you include other skills into your stories so the magic becomes part of them and not something exclusive.
Try and remember how it felt when you were a child, what entertained you? What mattered to you? Another thing I'd say is don't forget that something can be magical without it being a magic effect, for example to a child some glitter thrown in the air as fairy dust can be as real and amazing as a brilliant sponge ball routine.
If you can afford a DVD investment I would definately recommend you buying
Perform Like A Pro by Quentin Reynolds. This is a lecture from The World Magic Seminar on this exact subject by a fantastic childrens entertainer, with over 30 years experience. It can be found at the link below or (occasionally on ebay). It will help you to build your character and in my opinion is an invaluable resource.
http://kidshowmagic.co.uk/product_info. ... 3c04b57784
Hope this helps,