The kids can use your account to listen to free music like from Prime (assuming you have a prime account), tunein radio, Pandora, etc. and if you bought the unlimited music plan they and you could listen to anything from Amazon. A second account is an unnecessary expense, as it would have to have the $100+ Prime account, too.
Why not put the Echo in a central location for all to use, and get yourself and kids Dots (which can cast its sound via Bluetooth to speakers, etc.