I'm taking my son to two Agenices this week and they asked me to bring a headshot with us.

What Size should that be?

typical is 8x10 black and white (sometimes - 5x7... but not typical)

8 x 10 or 11 x 14 should do the trick one color and one in black and white. Make sure you take other body parts just in case they want to see more of the kid and not just headshoots. Good head shoots are from the chin up or mid chest top of head.

8 by 10 is standard for actors, kids can get away with a 5 by 7.

word on the street is that 9x12 is agency standard

I think that the standard for a headshot that you hand in as a single sheet is 8x10 inches, but for a portfolio that you carry around to show ("book") the standard for models is 9x12 inches.  Photographers are expected to be a little crazy, so there are fewer standards, but many use 11x14, though some use 11x17.

I believe 8x10 color head shots are the modern standard for actors and "talents". Other than a few market that are still hanging on to the traditional b&w format, all major markets have move to color prints.

Models usually have 9X12 books, actors actresses 8 1/2X11 and kids there is no set size (at least here). Most agencies just want to have a photo of the child on file. Because kids change looks so fast it's not really important of the size of the image, it's more important that it's current. I shoot a lot of kids / babies and never, ever, has one been hired for a shoot without a casting where we can see the child in person within a couple of weeks before a shoot. With you also being a photographer it should be no problem to keep the images current. B&W or color does not matter. Because children change so fast be careful to not sign up for services that are not updated often. There are a few "agencies" that list kids that I would stay far away from (not sure about your area). If you want to, you can contact me and I will get you more info on it. Most people have a soft spot about their kids, and there are people that will prey on it. Good luck, watch your wallet. Most agencies are totally legit, but some can pull at parents heartstrings.

8x10.