Re-sizing patterns

I want to knit a zip up cardigan from one of Zoe mellors books but the biggest size is 3-4. how would i go about sizing it up for my 5 1/2 year old?

Having small kids myself, the biggest difference I can see in a 3-4 year old vs. a 5 year old is that the 5 year old is taller. So what I would personally do is just make the body of the sweater and the sleeves a bit longer than the pattern suggests. That, or use a slightly larger needle size.