I have to say I agree with the above poster. If you didn't have children, I would say you should do everything possible to try to work things out, but if this dog cannot be trusted around your kids, then the ONLY responsible thing is to find him another home.
If this is a purebred dog, you can contact a breed rescue group that will help evaluate him and place him (if possible) in a new home. But I do Great Dane rescue, and I have to tell you that as much as it hurts us to do it, if a dog is too aggressive we will recommend that he be put down. There are too many dogs with great temperaments that need homes as it is, and an aggressive dog is a tragedy and/or lawsuit waiting to happen.
Your dog is still young, so if you do decide to keep him he may still be trainable, but I would NEVER leave him alone unsupervised with your children. If you do decide to keep him, the first step, if you haven't done so already, is to have him neutered. This can help with some of the aggressive tendencies. You may want to look up on the internet "doggy bootcamp" it will give you some tips on how to deal with an alpha dog, which it sounds like yours may be.
Things I can remember off the top of my head are never play tug of war with your dog, as it makes him feel like he's "competing" with you, and never let him sit on your furniture, sleep on beds, etc. Doing so only makes him feel as though he's "equal" to you and your kids. He needs to know his place in the pack.
Hope this helps, and good luck with your decision!