My 10-year old has been up Si at least a half-dozen times over the last few years, but we always seem to go when the Haystack is very wet, or covered in snow. Yesterday we headed up and bagged it.
We left the Little Si TH midday and go to the first lookout point on top in 1:45(!), and then continued straight on to the scramble. As I wanted this to be safe and stress-free, I had brought harnesses and a 30m glacier rope. I climbed up 100 feet only to be a bit short of a nice tree anchor, so my son "simul-solo'd" with me about 20-30 feet before I threw a cordelette around the tree and brought him up.
"Pitch 2" got us to the ridge, and "Pitch 3" to the summit. It was good fun and practice/intro to multi-pitch "climbing". :-)