5th Ave Books is a large used book store in Hillcrest. They are amply supplied with children's and YA books, and are very well organized by author's last name. I have shopped here for specific books often, and am usually pleased to find them. The clerks are extremely well-read and can usually direct me toward exactly what I am looking for. I also tend to come across treasures I wasn't looking for- like this Gangsta Rap Coloring Book.
The only slightly negative thing I have to say is that they are a bit pricey for used books. While Baras Thrift Shop will sell you any paperback book for 50 cents, be prepared to pay $4-6 for paperback novels here. It's still cheap compared to new books, and worth it to shop somewhere that is knowledgeable and well-stocked.