Hi there,
If you are unable to climb Mt. Fuji this time, I would suggest going to Kawaguchiko instead of Mt. Fuji 5th station. You can see a beautiful scenery of Mt. Fuji from Kawaguchiko and also, there are a lot of hot springs with Mt. Fuji view. Besides that, you can also go biking around the Lake Kawaguchiko if you rend a bicycle with a child seat.
http://fuji-bb.jp/welcome/
http://www.fujisan.ne.jp/index_e.php
Kawaguchiko allows you all kinds of outdoor activities, so I would highly recommend there. On the other hand, there are not much to see or do around Mt. Fuji 5th station if you are no climbing.
Hope this information help.
Have a nice trip!