Young James Bond or The David Walliams books for the ten year old? Mallory Towers for the seven year old? a different Enid Blyton Book for the six year old? Mine love the Enchanted Wood and all the other Far Away tree stories.
I love Rapunzel's revenge by Dean & Shannon Hale for the 7 year old and Calamity Jack by the same authors for the 10 year old. Or The Savage by David Almond if he 's quite grown up. I'll have a think for the other one.