So I just grabbed my free copy of NWN and my kids are super excited because it has multiplayer. I have several PC's that are already connected through my home network so I can easily setup a LAN game for them. However NWN uses a key system for multiplayer, so in order to make this work I would need a unique key for each PC. I was going to purchase another copy (or two) of the game so everyone can play, but GOG won't let me. I'm guessing this is because I already have the game linked to my account. Is there a way to just buy the additional keys? I suppose I could set up some alternate "dummy" GOG accounts, but that's not really ideal.

No, you can't have more than one copies of a game in a single account. You could try asking support for additional serials if you'd rather avoid having to create an account for each of your family members.

The only real reason to have multiple Serial numbers would be if you wanted to do multiplayer, since I recall it will block or refuse if the key is shared with another game in session.

Although I know there's lists of the game keys elsewhere which you could use multiple copies of in order to locally get the game working (like I have, annoying trying to play with my GF with the physical disc and it barfs at me).

Make an account for each of your kids, then grab the free copy from the front page for each account. If you still need more copies, there should be plenty floating around in the forum.
Alternatively, assuming you'll be playing only on LAN, a keygen may work as well, if you wish to go that way.

Thanks for the feedback everyone. The idea of setting up multiple GOG accounts sort of feels like cheating, especially if I do it while the game is free. But if the other option is to use a crack or keygen well that's not any better, morally speaking.

I'll mark Ghorpm as the answer since it's the first and everyone else is essentially saying the same thing.

Alternatively, you can just save the gift and wait until you meet the person you will play with in the future. I owned NWN too and when i clicked the free button, GOG sent me a gift code with expiration date to NEVER. So I moved that mail to my "GOG- Gifts" folder in my mail and will give it to someone who needs it in the future.

I said this because I am assuming at the moment you are doing this just incase. If you already had a friend you are planning to play with, they would have gotten the free copy themselves. By keeping the email without redeeming it, you preserve to option to gift it to ANYONE you want in the future instead of it being stuck into one account.

Why would it be like cheating? Me & the wife usually play LAN games and we both each have our own GOG account, NWN being one of those games. If we want a gog game, I purchase two copies (actually that's how we got NWN, we bought it as soon as it was released here) so I don't see any problems in getting a game for free for each account when the chance appears.

Think of it this way, one day your kids will emancipate and they will need their own gog account, right?

Also, I bet Gog staff is totally ok with this. I have a few friends who don't have a gog account and I emailed them as soon as I saw the offer. This is very likely a tempting offer to attract more people to the store, which to me is a perfectly fine business strategy.

It makes me feel like one of those kids who goes trick or treating with extra masks so they can visit the same homes over and over.

I went ahead and created accounts for my kids, though I had to lie and say they were 13 or older. However, after thinking about it over night I don't feel too guilty about it anymore. This whole thing could have been avoided if the digital rights to this game weren't managed this way...GOG.

Creating the accounts and letting your kids use them are two different things. ;) As svmariscal kinda said, they'll have the accounts waiting for them when they get older. And there's really no need to even use the accounts for anything other than retrieving the individual multiplayer serials.
And what do you think GOG could realistically do about the requirement for user-account-linked serial keys?