My newly adopted Saint Bernard attacked my other animals...Help!?

Definitely sounds like resource guarding issues. My collie has the same problem, she was also chained by her previous owner. Dogs that are chained are more likely to end up like that for some reason..

Was either the cat or your other dog harmed or just frightened in the attack?

If either of them were harmed, I would say that probably it is not a good idea to have that particular dog around small children. If neither of them were harmed, then I would suggest being very careful about leaving anything about that he could consider a food item to guard. Some dogs only resource guard from other animals, that's the case with my collie. However, I wouldn't trust her to understand that a small child is not another animal she can boss around about food.. Another thing that needs to be done is that your children need to be involved in giving the dog commands. You have to establish that your children outrank the dog. Perhaps you can practice just having your 5 year old tell the dog to sit with you standing there to help? The ten year old could probably give him commands like sit, stay, heel..etc..

Me personally, I wouldn't trust the dog unsupervised.. He's just too big where he could hurt them badly by accident. At this point, before he's bitten anyone, he will be a lot easier to rehome. I would suggest maybe searching out a breed specific rescue as normally it's a bit of a wait to get them into rescue..

I would NOT trust that dog around anyone or anything. I have known a lot of them with the same exact history you have described. I am not suggesting he be euthanized but he should be with a family with no kids or other pets. And that family needs to be made aware of his dominance/aggression factors. And he needs to be in a serious one on one training school with his new family.

Someone is going to be hurt and it is not even that dog's fault. He is just the way he is. But someone needs to wake up quickly before it is too late.

Best of luck.

Well I guess it was the bone. I remember the first time I bought a nice bone for my dog, he was so selfish. I poke the bone with a chopstick and he just snapped it with his teeth. About your kids, just tell them to stay away from the dog when he is chewing his bone. It wants its time and no one to disturb. If this all happened after the bone, try removing it from him for a few days and see if there is any different.