I have this same issue with my kids account. I just have them play as guest. I thought about doing the copilot thing. The Xbox just came out with it. You can link the second controller to your and then I just have to login once and the kids and I can take turns playing. The drawback is we have to use the same character. I honestly like this better than split screen though because even with my 43inTV it's hard to see what's going on in split screen.
I figured this out and I felt kind of dumb for not solving it sooner. When you play with your kid you get the message when doing a split screen about signing into EA. Even if you created an ID for your kids with your computer it may not work on this screen. What you have to do is click create at that EA screen asking you to login after choosing split screen. Then it will ask you for the PARENT's email address. This should be the account you have your main console Gamertag linked to. Create a password for the kids account and it will create an ID for them. After that, you can both play split screen without them playing as a guest. They will also be able to play the game without you having to be there to sign them in. I hope this helps.
Good job, I'm glad you got it worked out. I'll need to remember where this post is so I can reference it for others.
