the creator of the best-selling alex rider spy novels has accused fellow children 's author david walliams of dumbing-down fiction and failing to ` challenge ' young readers .
anthony horowitz said books by the comedian .
who was the uk 's top-selling children 's author last year , are ` witty and entertaining ' but nowhere near .
the 59-year-old novelist and screenwriter singled out walliams ' gangsta granny for criticism , as well as the diary of a wimpy kid books by author jeff kinney .