I would advise you to think of photography as a print-making craft (even in this digital age). By this, I mean think less about what your subject is, and more about how the final print will look. Think abstractly about composition, tonalities, colors, etc. Notice how your eyes naturally move about the final print. Powerful images do not require powerful subjects. It is all just light. From your subject list, I would start with flowers (using your tripods), being the slowest moving of the list. Once you have mastered slow deliberate compositions of light with a static subject, your eye will know how to see the same opportunities with your swiftly moving grandkids.