A .22 will be plenty to put down a rottie. I have killed scores of 250 pound hogs, and a few cows with a .22 and .22mag. Picture and imaginary line that crosses between the base of the ear and the eye socket. Do this with each ear and eye. Where the two lines intersect, is the magic spot. He may yelp once, but it will be lights out, and he'll be dead on his feet before he can blink.

Shot a 250 black bear with a rifle in the same spot this fall, and his head was like a bowl of rice krispies. If you use a caliber higher then a 22, be careful shooting him at close range, or you may end up getting hit with fragments of lead after the bullet passes through his gourd. If you use a rifle, you are better tying him up, and greasing him with a high shoulder shot. The impact will shut his nervous system down pretty quick.

If the dog did that to my kid, I would have zipped him ASAP, and not given a rip about his feelings. As much as I love dogs, I have no tolerance for them ripping on my kids. Do not give this dog to someone else. You are just asking for someone to get hurt/killed, and getting your rump sued. Especially if you know that this dog has attacked someone already. Hate to sound like a mercenary, but unfortunately, I have sent lots of critters to meet their maker. Good luck, as I know it won't be easy.