I have had 3 under 5
It was ok really, HE-wise. I tend to see anything done before they are 5, or even 7, as a bonus really.
Your kids will probably have a BIG advantage, actually. I expect LOTS of play goes on in your house? I tend to think that the single most important thing for under 7s is lots of play, preferably with kids of a similarish age. A little younger or older is good too, as they get to explain/discuss stiff/be explained to by someone in the process of understanding things themselves.
My experience is that if they have worked things through via play, then they will learn far more effectively, and quickly. It is extremely common for home educated children not to learn to read until quite late, say 11, but then be reading ABOVE age-average within a short space if time.
Re reading, that is up to you and your children. When they are ready, it will become clear, they will start asking what words mean, trying to sound out letters, etc. They then might want you to actually teach them, or they might want more time figuring it out.
I'd read the new alan thomas book first, and/or go along to your local group.
good luck!