[Cynthia]

It's normal for children (and even adults) to be afraid of dogs that they don't know, especially huge dogs. Don't try to force her to interact with the dog. Reassure her that the dog won't hurt her and comfort her when she's afraid, but I wouldn't ask to have the dog put outside or away from her.
Dogs that big have a tendency to knock kids over without noticing. Mastiffs also tend to lean and stand on people's feet. Could this have happened to her?

 

[Cynthia]

I have been thinking about my post and would like to amend my comment above. The dog can hurt her. I'm not sure if you meant 160 lbs. or 260 lbs., but either way, these dogs can easily knock down a small adult, even more so a small child. That could cause a child to get hurt. Also, depending upon how well trained the dog is, they can run someone over quite easily, so you have to watch out for them. I have seen children unintentionally hurt by these dogs. It's never been serious, but it could be, if a he were to knock her down the stairs or against something.

Therefore, I wouldn't tell her that the dog won't hurt her. I would tell her that the dog is friendly, but he's a dog, and dogs don't really understand like people do. It's important to be aware of him, so she doesn't get knocked over. It's probably still going to happen though, because dogs and kids.

 

[Cynthia]

The English mastiff is exactly like the border collie in every way in that they are very sweet and would never ever hurt anyone. Also the border collie is a lot bigger than my little girl and has the potential to easily knock her to the ground with no effort as well, but he would never ever do that. The dogs are all super sweet.
I’m just a little confused because my daughter immediately started playing with the border collie as soon as they met, but she wanted nothing to do with the mastiff even when they were meeting each other.

Dogs don't think like people do. A dog can be as sweet as could be and still accidently knock you over. I've been around quite a few large dogs and they can't pay attention the way a person can. They lack understanding.