DS was a precocious reader and although the school were absolutely horrified (to put it mildly) about the level of his reading when he eventually started school; I know it was the best thing that I have ever done for him.
What I would say though, is that it is important that your DD learns to read via a phonics method, rather than a sight method, as there could be difficulties with spelling later if not.
We used Jolly Phonics and DS was enthralled by the DVD. I then bought a set of Usbourne phonics books from the Book People, I think they were all at the same level, but as he had such a good foundation of phonics he never read another graded reader. He then progressed straight to Dr Seuss and would sit reading to himself giggling his head off. Friends also bought him a few audio books. As he was turning 4, he was then reading Horrid Henry, Winnie the Pooh, Beatrix Potter and all the ladybird classics, Snow White, etc, along with his favourite Thomas and friends and Mr Men books.
At school and home, his party trick when forced to read anything boring, was to read each word phonetically backwards.
Roll on a few years and by 8 he had the reading age of a 17 year old and absolutely adores reading - do not let anyone tell you that teaching a child to read early will put them off. On the contrary, it has opened his eyes to the world around him and he enjoys a good debate on a diverse range of subjects. I have tried, from time to time, to limit the reading a bit, but he was able to have a very mature conversation about it and told me how much enjoyment he got and how he could not imagine a world without books.
She is a little young yet to learn writing but I would also suggest that you start this as soon as she has the fine motor control, as ideally, you do not want a huge gap between reading and writing.