Knowing this fact does not make it any easier for you to chase after a nitro-charged 3-feet tall monster. But after thinking hard about why I am bummed-out even before I start my work in the morning because I have to rush the kids to school, I came up with this trick that may just work.
Let's pretend you are a car running on 50 liters of gas. And your kid is actually a bunny running on energizer batteries (the long lasting type). For the sake of argument, let's make him run on gas too and that his full tank is a mere 3 liters. Although a direct relationship is difficult, it's easier to work things out if you imagine that your tank capacity equals your age. Ofcourse, this does not work for a 77-year old grandpa. I am assuming that you have not yet reach midlife crisis and that you still eat a healthy meal. OK, back to our story. Altough you have the bigger gas tank, you have to unfortunately lug around a lot of weight too. In comparison, a little child has a smaller tank but his weight is much smaller. One good number that comes to mind is mileage. A sub-compact gets so much more bang-for-buck compared to a 3.0L grand tourismo class vehicle. So that little car can just zip around town without burning too much fuel. The same applies to people too. Although kids are no good for 20km marathons, they are deadly when confined to a small room for anything longer than 30 minutes. So how do you handle the 3-year old kid. Just confine your movement to sporadic quick bursts.

Try it and let me know how well you fare.