They're not afraid of printing big books for younger kids. Some very popular books have large word counts...
A guide for popular books in each age range (that you've got correct there)
Beast Quest = 10k per book, that's for 7-9 year olds
Derek Landy looks around 60-90k, 8-12
Rick Riordan 60-90k, 10-14
Darren Shan 50k, 12+
1-5 is going to be picture books
5-7 is Andrew Cope, Daisy Meadows, younger Enid Blyton stuff etc
8-12 is Beast Quest, Derek Landy, Michelle Paver, Michael Morpurgo (massive word count range for his stuff), JK Rowling, David Walliams
12+ generally domatined by vampire fiction at the moment; also Michael Grant, Robert Muchamore, Suzanne Collins etc
Edit: So pretty much take your lower word counts for the ages given as the minimum, and anything above that up to full novel length is accepted and currently being taken on