i don't have kids, and so have no first hand knowledge here. most of my second-hand anecdotal experience is that people have kids, and they often still have ideas, but no time to really implement them.

i did have one coworker at my old job who said that the limited free time he got once he had kids motivated him to be more productive, and actually made him better at getting side projects done. probably the exception, and he was a pretty motivated and focused guy anyway, as far as i could tell.