Why don't you talk to him about the two of you setting very firm times to play? No guilds or raiding allowed so nothing pressing him to be at a raid at a certain time, etc (he doesn't need that high end gear more than quality time with you and the kids), and maybe even play with him again, but ONLY during those times. I take back the suggestion of your playing the game with him if it was addicting for you when you were playing.
The awful thing about online games is they don't end. Unlike Legend of Zelda or Mario Bros, these go on and on until he gets an invite to his childrens' graduations! I totally know games are addicting and do not mesh with time for the family.
I do not believe the answer is treating him like a child. I firmly believe in the long run (if not in the short run), that will be detrimental for your marriage as it will put a wedge between you. After almost 22 years of marriage, my husband and I have learned to respect each others interests, and we try to get involved in different ways so we can share them together. Again, game time can be addicting, though, so this is a little different than another hobby, but both sides need to be understanding and okay with compromise.
Getting the family out of the house and away from the computer, like someone mentioned, is a great way to get him to see again how enjoyable family time is as well.