As an owner of two large dogs, I find it admirable you're taking some responsibility for what happened to your dog. Many pet owners would be quick to blame the other dog, even though it was on a chain.
However, incidents like this don't need to happen. Large dogs should be behind fences. And securely behind them, at that. Dogs that are chained up often times are unhappy animals. If your small dog got within range of this other dog, chain or no chain, the other owner is partially responsible too. What if a child had wandered in?
Anyway, I'm glad your pet will be well .. At very least, it should stand as a real learning experience for the children that left the door open. This is how we learn in life.
