Riven - don't think to yourself that daughter is a bit behind!! Have you looked at the Early Learning Goals??
You cannot assess any child's reading ability from what level of commercial reading scheme they are on!
Does your DS school teach a phonetic method of reading???? I ask because ORT is based on sight vocabulary - recognising words from memory, and has very few phonetically regular words (words you can sound out like cat). A few years age this was the way that reading was taught and most schools have a big stock of ORT. Now things have gone back to teaching reading & writing using phonics. However, reading books are expensive and schools cannot afford to replace ORT with a more phonetic scheme so often keep the newer books for sharing in class and send home the ORT.
We use book banding in my school which is farly common in most schools now(all commercial schemes & real books are grouped according to difficulty) This gives a more reliable overview of where children are at with their reading skill development.