Before you have kids, figuring out the value of something is very straightforward. It's worth time, or money, or enjoyment, or some combination of the three. You can simplify a lot of things by figuring out if doing something/having something/making something/being something has value to you. If it does, do/have/make/be it. If not, don't.
But once you become a parent, the whole concept of value changes. You find yourself doing all kinds of things that wouldn't have been worth anything to you but are worth something for your child. Driving long distances, paying for private schools, spending time on projects you can't stand, endless rounds of "The Wheels On The Bus," chaperoning One Direction concerts. Listening to long, elaborate, narratively dubious dreams. Picking the spoon up off the floor again. Showing up at the rink for practice at 7 am on a Saturday. It's a seemingly endless list of things you would never in a million years have done, but now you do with joy. (A cranky joy, sure, but joy.)
Investors can calculate the Net Present Value of any potential business venture to decide if they should do it or not. The Net Present Value of activities for parents is simple: Does it have value to or for your child? If so, then the NPV is higher than the opportunity cost and you do it. Gladly.
(If you find the topic of value as interesting as I do, join me on my new Twitter feed that only talks about value, in all sorts of contexts: http://twitter.com/valuestan.)