You need to decide what works best for you and your family. If the account is shared they can use all the same apps, watch the same movies and read the same book, etc. If there are separate accounts, everything will have to be purchased twice.
While it's not difficult to switch between different accounts, it's not something I would want to do all the time. I have 2 accounts, one for my husband and one for myself, with the exception of just a couple of books everything is in my husbands account because changing back and forth is more than I care to deal with.
I know that on the older Kindles the books stay when you change accounts but I've heard that's no longer the case with the Fires (new to Fire too so I'm not sure).
Have you looked at Kindle Free Time yet? It looks like the perfect thing for the age of your children! https://www.amazon.com/gp/feature.html/ref=kin_dp_ftu?docId=1000863021
While it's not difficult to switch between different accounts, it's not something I would want to do all the time. I have 2 accounts, one for my husband and one for myself, with the exception of just a couple of books everything is in my husbands account because changing back and forth is more than I care to deal with.
I know that on the older Kindles the books stay when you change accounts but I've heard that's no longer the case with the Fires (new to Fire too so I'm not sure).
Have you looked at Kindle Free Time yet? It looks like the perfect thing for the age of your children! https://www.amazon.com/gp/feature.html/ref=kin_dp_ftu?docId=1000863021