I have learned, from having 2 kids into MTBing (my fault, by the way - I got them into it!) is that it's a lot more expensive than it looks. I've learned that "neat freaks" don't belong on the trail maintenance crew. That one should never leave home without a few plastic bags for muddy clothes. That one should never leave home without a space blanket, spare towels, shirts, pants, and extra food & drink. Never mind spare tubes & assorted parts & tools.
Mostly, I've learned that mtn bikers are pretty nice folk, and i'd rather spend my time & $$ having my kids hang with them than playing football.........
(I've also learned that putting an XC race on is a PAIN IN THE A#%, BUT, THAT'S OFF TOPIC)