In spite of the anti-cross-breeding sentiment often shown, it's interesting that it's exactly by doing that that has brought about several breeds we know and love - like dobes, APBT's, ACD's and many more I probably don't know of (in otherwords, all hail the mutt!)....but that's just my little 'aside'.
On to the question: The Great Herding Hound!
To keep my kids out of harms way, I'd go for the herding instinct/intellegence of a border collie (to round them up when required) and the size of an Irish Wolf Hound to intimidate any future boyfriends....not to mention a dog that size could carry my tool belt or camera bag without passing out.
"These things happen.....to me." - Ron Stoppable