I bought "The Last Of The Really Great Whangdoodles" for a niece and nephew recently. The former was nine at the time, the latter eight, iirc. Jolly good feedback! I read it through a bit first, and it certainly seemed like fun. She's written another, called "Mandy", that is also supposed to be rather good.

"Storm Warning" is pretty good, but it's also really old. They've come on in leaps and bounds since then. I'd recommend trying one of the more recent ones. "Blood Of The Daleks" perhaps. That was the start of a new series, with a new companion.