School kids have photographs taken every year. If your children are not in the school system, you will not have a built-in photographer every fall.
It is expensive to visit a professional photographer (think senior pix), but there are ways to have photographs done on a budget. Kmart, Wal*Mart, and other discount stores often have studios where you can get a package of photos done for less than $20.
Even better, with the availability of digital cameras, you can easily take your own pictures. Take twenty pictures, delete all the ones you don’t like, and keep the best one. Crop it on your home computer (set the constraint to the size picture you want printed), upload the file to a developer (Costco, and many drugstore chains), and have the pix mailed directly to your house. The last time we did pictures this way, it cost 39 cents per 5×7 (we got one for ourselves and one for each grandparent), and 39 cents for a sheet of four wallets. For less than $2 per child, we had a nice set of basic school pictures.
Tip: take a look at many school photos. An old high school yearbook can be a good source. Note that more of the body shows on younger kids. As kids get bigger, it is common to have the photo subject turn at an angle instead of doing a straight head-shot. Having the shoulders turned makes the person appear smaller, while having shoulders square to the camera adds weight.