How do you prepare a child for their first headshot session? This is a difficult question because the answer differs based on how old the child is.
Typically, if the child is very young, there isn't too much you can do except make sure they look their best. They shouldn't be sick or hungry on the day of the session, and they should be in a good mood. Remember that to get good headshots, mood is everything. If someone comes to a headshot session unhappy, their photos won't turn out good unless the photographer really knows what he's doing and can lift people's spirits quickly.
Now, if the child is a bit older, such as between seven and twelve, it's a little more complicated to prepare them. Instead of simply making them happy, you have to do all of that in addition to making sure they follow my clothing guidelines and that they prepare some different looks.
Remember that if you're child is very young, they shouldn't try to play older characters as their main headshots. They can do it for fun because it's cute but they will likely be playing a kid role. For this you just want to be sure that they know what they are planning to play and that the photographer remembers to get some happy and some serious looks for their shots.
The most important thing though is that they come in looking happy and excited for the session because you can't get that session time back.