As PP said there are as many different ways to HE as you can think of so whoever you speak to will most likely give you a different answer. Its all about offering flexibility to both the children and the parents. For instance, even if you choose to follow a curriculum, you can choose the times/subjects/speed to suit your family's/child's needs. You can even choose to return to school! THat's the beauty of home ed - YOU CHOOSE
No one can say its too early/young to be learning something. We are born to learn and if she's enjoying it then surely early exposure can only be positive. We find when learning new concepts that we start it off, play about with it for a while, then leave it and come back to it later. Had to do this with DS's reading, left it 6 months in the end and he came back to it really well. His mind had been continuing the learning the whole time we 'dropped it'. DD2 & 3 both started phonics around 2/3 and have got on well with it. I think one thing that is important to hold on to though is an interest/love of learning so trying not to overdo it when they are little/the concept is new. Its a great adventure, hope you have loads of fun and get loads of advice/strength from here.
As far as GCSEs go. DD1 (11) wants to do hers so we will be choosing 3/4 for her to concentrate on. We haven't got that far yet though, got a bit of time yet. I don't really know much about the whole process though...