I don't think I've ever done anything other than 10, and I just recently used up my last bar from one of the first batches, almost 3 years ago. Very hard bars, no DOS, lather nice/creamy. I have never used a preservative, so I was really surprised they lasted so long. I mostly use EO's, so not a lot of scent left after so long.
See, as for baby/children's soap...I've used my 10% SF on my sone since his very first bath; he's turning 3 next month. My 6 year old and my almost 3 year old both have very sensitive skin. My daughter was almost 4 when I finally trusted my soap enough to use on her and her eczema/psoriasis went away after that. My son has such sensitive skin that if he stays the night/takes a bath at my MIL's he'll get a little irritated on his bottom/face/sensitive areas.
Hope that helps!