Ah, my favorite type of track building questions! As long as you have a good adjustable power supply, kids can drive almost anything. I've had my 4 year old nephew on my track with no issues one the power is adjusted to his ablilities.
Turns - varied corners make a track truly fun to drive IMHO. It isn't necessary, but I like the way they drive and look from a scenic perspective. This is how different each corner on my track is:
You can also see that I'm addicted to squeezes in the above picture. Mine are setup so the two outer lanes can always pass. The inner lane functions as the blocker/blockee in this case.
With kids on the track and the power down, I simply either get ahead or behind them in those areas.
As Bibbster says above - elevations can make some cool changes. I have over a foot of change from one end to the other. No two turns are on the same elevation. Check - that - actually at this exact moment they are ;)