Well,
I'ts been a long time since I rode,and secretley miss it!
But for beginning riders,I guess the best advice is not to try to learn too much too fast.
Practice just a few obstacles at a time,over and over, until you are confident at that, then move to the next.
Learn to always ride in an attack position with your head up over the cross bar and your arms in a shock absorbing position. and sit down only in the corners. later on, more advanced tecniques might include learning to "bunny hop" your bike... in other words squish down the suspension and unload it for maximum flight.
Never follow directly behind another rider, you will never pass that way,also you will get taken out if he goes down.
On small bikes (125 & under)learn to keep your momentum throughout the corners (rpm's up) so the engine doesn't lug down.
accelerate up until the last possible moment in corners,use both brakes hard and accelerate asap all the while keeping two fingers on the clutch lever to work the rpm's or avoid a stall.
It is so great watching your kid race! I loved watching my kid rack up the BMX trophies all the while living vicariously through him!
Run and lift weights as it is sooooo important to be in better shape than the competition.
GREAT STUFF!
Good Luck....Hard work equals more luck!