Our two best ancient sources for his life, Suetonius and Plutarch, both begin in his late teens, unfortunately, so there isn't much information on his childhood. Both of those are pretty short reads, and I think you can get a lot out of them, in terms of personality.

There's also Sallust, who actually knew him. The linked work is a history of the Catiline conspiracy, but starting at paragraph 54 he speaks at some length about the contrasting personalities of Caesar and Cato.

If you're looking for something longer or more recent, probably Christian Meier's Caesar spends the most time on assessing his personality and psychology, although I really don't care for the way he doesn't footnote much, and some of the conclusions he comes to are rather...weird. But it's definitely an interesting read.