This thread wnt off-topic into a different discussion altogether, but let me try and address some of the raised issues:
Is it permissible to have two accounts in the same household?
Yes.
Two family members would have individual accounts on their own computers but sharing the same ISP address.
Not a problem.
Account Sharing:
This is indeed a bannable offence but there's more to this most might realise. In terms of the OP looking to play with the grandson, there isn't any problem whatsoever.
Problems occur when Player A shares account info with player B and then we find player A has a ticket at Customer Support claiming they've been hacked and all their gear is missing.
Upon investigating, it is found that player B has had routine access to the account of player A and it is this player that has unbound all of the gear and sold it to gain gold.
We take Account Security very seriously and in the example above, player A will have to live with their loss because they shared the account info.
As for the OP, I can acknowledge that Grandparent might have knowledge of the Grandchild's account info but there shouldn't be any need to abuse that knowledge.
Let me ask you this:
How many of us know the username and password to friends/parents email, facebook. whatever-else? Do we go in there and mess about?