I have to start with this - unless the people at the daycare are in a 1 on 1 with every dog there, in my opinion daycare are the last place that any Pit Bull should be. Ever. One minor scrap between two dogs can quickly turn into a rolling battle with every dog in the room involved and no matter who started it or who is involved, the Pit Bull will be blamed. And having broken up a spat or two in the past, two dogs going at it are difficult for one person to break up. I can't imagine trying to get a bunch of dogs apart even with two or three other people.
Also, there are many threads here about how to break up a dog fight safely. Please do not hit your dog to do it. Many people swear by break sticks and other methods, but personally I think the very best way to do it is to simply lift the dog by their collar. It may take a few seconds longer, but in my opinion it's much safer for the people and the dogs involved.
Forcing / coaxing a non-dominant dog to act dominant can be a setup for worse things down the road. Likewise, holding another dog down to allow it can also lead to serious problems. In our house, we decided on the "pecking order" of the dogs and when we don't keep up with it we have behavioral issues. Granted, we don't have fights, but Connor starts marking all over the place.
I'm not trying to be nasty, please don't think that. There is a LOT of advice out there, and if you put three "trainers" in a room, the only thing that two of them will ever agree on is that the third doesn't know what they are talking about.
The leash could have played a part in Mia's sudden move, as leashes can increase a dog's defensive behavior.
If it were me, I'd give them neutral interactions in neutral places. Don't force them to interact, and don't have toys, food or anything else involved. Even our boys (who would happily kill each other) can be around each other leashed and behave themselves. They don't have to be "friends" but it's not unreasonable to expect them to behave either.