I was advised at Snow and Rock that Gore Tex jackets for children are not made. Who wants to pay £250 - £400 for a jacket that a child will grow out of in a year or two?
Here is a good description of how waterproofness is measured.
Everything you need to know about Waterproof Jackets, from hydrostatic head tests to crucial features & types so you can choose the best jacket for you.
www.mountainwarehouse.com
Don't make the mistake I made many years ago. For Christmas my brother bought me a brand new North Face Gore Tex jacket. What he didn't tell me was that he picked it up in a Shanghai market. Confident with the jacket I strayed out onto the very wet Plynlimon Hills. It turned out that the jacket was just about as waterproof as a fishing net.