For example, each lens loses some of the light entering it, so if you set the lens to f/2.8, for example, after the light loss, it might be f/2.6.
You need to know the "T" (transmission) stop for your lenses if you want to be as accurate as the in camera metering.
Sorry, but it would actually be a larger number, so a f/2.8 lens, after light loss, would be an actual t/3.1.
As for the parent, you should read up on what a histogram is and how to read it correctly, you'll benefit a lot from that and you'll be able to make do without a lightmeter for a while (unless, as it's been said, you do studio work). Read up on your camera what the exposimeter does and the different metering zones.