I also have a power chair and have found that it has made a tremendous improvement in my quality of life. I'm able to get out of my apartment more and be more social/get around.
It's a little difficult to say exactly what is the best thing to do. On one hand, you want to use the chair to reserve your strength and stamina so you can play with and take care of your grandsons. However, on the other hand, you also don't want your muscles to completely atrophy and get so weak from lack of use that you end up 100% confined to a wheelchair.
What I typically do, and please only take this as my own experience - is try to use my chair as little as possible when I'm in my apartment so that I am getting at least some "exercise" and mobility and definitely use the chair when I go out. Of course, this also varies depending on how I'm feeling - if I'm having a bad pain day or feeling especially tired or weak, then I definitely use the chair more often - both inside and out. It also varies depending upon what I'm doing (i.e., how much energy the task/activity takes).
FurballsMom gave some good advice also about using the chair when your grandsons are most active so that you're not exhausting yourself trying to keep up with them and then maybe "giving yourself a break" from the chair when things are a little more quiet.
Basically, it really depends on how you feel - if you are at all uncomfortable or scared about walking, standing, etc. - or afraid of falling - then by all means keep that chair close by and use it as much as you need. After all, that's what they are for!
I would at least use the chairs when the kids are at their most active, in order to have enough energy and strength.