Leslie,
I always hike solo - think it is the reason I get such immense satisfaction from the Sierra. One of the few situations in life where every decision is yours - there is no one else to credit or blame for the adventure.
Having said that, to me the real risk of a young child on the PCT is the isolation in certain sections. Sometimes you are days from any kind of help.
Although the risk is small, bears, mountain lions and rattlesnakes are there. I’ve come face to face with a bear on several occasions, been stalked by a mountain lion (they tend to favor smaller females and kids), and seen rattlers every year. Once, in a late day hiker’s trance, I nearly (very nearly - probably set some kind of long jump record) stepped on a rattler minding his own business in the middle of the trail. Then there’s always the possibility of leg / ankle injuries - not as serious since you can just wait for help - although I’ve been on the PCT south of Forrester Pass and didn’t see anyone for over two days.
Enjoy yourself - just be aware of and evaluate the risks. The PCT north of Kennedy Meadows is pretty remote and demanding.
Booger