Difficult but rewarding. Every time I send my son out on the track,, I'm filled with pride that he wants to do what I do, and scared that he may fall, miss that gap on the 80ft triple, have the motor I built seize at the wrong time and cause him to go down and get tangled with other riders, or some other problem, Those thoughts are fleeting, just parts of a second long, but they are BIG in my mind. I focus on his riding, the sound of the bike, planning what will be done when he is don with the moto, or after the enduro. It's something worse when we are racing enduros or scrambles at the same tome, in different classes. I know he is out there someplace, and have to really focus to keep from being distracted from just riding my ride.
Prep your son well (sounds like you have), prep the bike well, set him loose. The anguish you feel for his safety is fleeting the joy you feel from seeing him ride will last forever.