Children biking

So I introduced my 10yo to a hill to bike down, and a shortcut it has to a local park.  She has since become obsessed with riding down this hill at top speed (to be honest, I have too) and then taking laps around the park's sidewalk.  This last week I told her she has to start biking up the hill each time we do that.  She can make it halfway before stopping, and about 3 times up the hill kills my legs to death.

My 7yo is starting to catch on to biking.  My approach has gone from constant critiquing (and him telling me that he refuses to go without training wheels) to leaving him alone and having him figure it out on his own.  We were using the same hill as above (just closer to the bottom) to teach the balancing without the need to learn pedaling and he has started to learn exponentially.  I even brought him on one of the park expeditions with the 10yo and he managed all right.  He averages pedalling about 7 seconds before having to restart, except uphill.

I think I now understand why people wear biker shorts.
1) Because pants hit the gear box
2) Because pants (particularly those on the tight side) cause pain in certain areas