My eldest is 11, and my youngest is 6 and a half-pint (like his mum!) whom I'm sure would love to sit in your engine bay and have a fiddle! :P
Whatever happens though, don't leave the keys in the ignition!
A couple of years ago my middle boy was 7 years old - my wife had got the kids ready for school and Connor (middle one) insisted on putting the key in the ignition on my old car - an E34 BMW 535 manual. The car was parked facing down our quite steep driveway, now because the handbrake wasn't very effective we used to leave it in 1st gear. In he jumped, put the key in, then turned it all the way around!
Car started and proceeded down the drive, over the road, straight over a pretty mature privit hedge and bouncing off the neighbours house
We don't let him any where near the car keys any more!
Tony.