As a photographer in Edinburgh, I end up spending a lot of time in various parks around town so I often suggest bringing a football or a frisbee with you to your photo session -- when your kids are a little older it give them something to focus on other than be photographed! And it allows your kids to just be kids and play a bit, with each other and with you. This photo session was especially lovely as the grandparents got to join in with the fun.
