One of the tricks to getting good candid photos of kids is to get down at their level. For most shots, the camera should be at the same level of the child's eyes. This sometimes involves getting on your knees or sitting on the floor (depending on the age of your children.) I see a lot of parents and grand parents that photograph their kids from a standing position, this creates a photo 'looking down' with the background being the floor or grass. If you get down to eye level of the child, I find you'll get much better photos. Give it a try.