10 yr old get Percy Jackson series (nephew mad on them) 7 yr old get Jack Stalwart adventures (another nephew loves them) 3 yr old ds loves anything with Shirley Hughes' Alfie in it, or the Little Red Train by Bendict Blathwayt
11 year old niece - well dd2 is 12 and is reading the Twilight series and prior to that read the Angus, Thongs series but I think there is quite a difference of maturity between 11 and 12 so neither may be any good. Last year she enjoyed all the Mates, Dates series, as well as The Edge Chronicles MuddleEarth, Black Beauty, Balet Shoes, Charlotte Sometimes etc.
7 year old niece - Pippi Longstocking or one of the 'older' Lauren Childs (have forgotten what they are called but they are fabbo) - or Judy Moody.
ooh just thought 11 yr old niece - get Star Girl, it is such a lovely book. Totally agree on Holes too - brilliant book. Skellig is a great book too for either 10 or 11 yr olds, don't think it's a girl or boy book, but is excellent