Using a green screen might be very useful and fun. Dave cross here put up a nice 8 minutes video on how to use a green screen for kids portrait photography and adding some effects using PhotoKey 5.
Dave uses here some Wescott green screen kits, that comes with the screen and the software, who I must say I didn’t heard about, but seem to do a very fast and good work when it comes to green screen effects.
Here is the video, I think you might like it!