Hi there!
Looking at your itinerary I think the best way to travel would be to buy normal train tickets.
A JR Pass is amazing if you travel between a couple of regions in Japan, for instance a return Tokyo - Kyoto will quickly pay off. Your itinerary consists of just one way Kyoto - Tokyo with a stop at Hakone (which is directly on the route).
A JR Pass would work very well if you traveled from Tokyo to Osaka and made the itinarary above but for now I would recommend going for normal tickets. What is nice to know is that children below 12 pay half price and children below 6 are free of charge. Some conditions apply, you can read more about traveling on the JR with children here.
I hope this helps!
Daniel