Unless you are actually video taping the whole exchange you would have no idea if they were wrong. It may also be that they put her in a diaper because the parent didn't provide extra clothes and that is what their policy is. Unless you have a discussion with a provider about what is going on, with no kids present, there is no real way to determine what is going on. I think it is silly to jump to the conclusion that the provider is just being an ass when it may be the parent who is causing the problem. From what I saw in daycare, when a child had a hard time adjusting it was either the parent or an illness. I also find it hard to believe that every other child in there does well except this one but it is the teacher's that have the problem and not the one family. If you talk to the other parents and they also have a problem then you should definitely not use the nursery any more and if you get a bad feeling when you talk to the provider than you should avoid it like the plague, but if the other parents and their children are happy with what is going on and the kids are happy overall when you drop in then you should look at what you are doing to cause your child to have a tough adjustment.
You should also make sure to give them a change of clothes and tell them under which circumstances you want to be called from the service to get her, it may be that many parents at the church don't want to be called out for crying or an accident so they don't call parents for those things. You need to make sure you know the rules they are following for things like that and tell them the adjustments you want made for your child and if they don't want to follow them then leave..
OP...I would definitely talk to whoever is in charge of the nursery and try to find out what their policies are and whether or not the worker was violating any of them. Please don't eave your daughter if you don;t feel comfortable.
I spent a lot of time in church nurseries and sunday school and I had some really bad experiences (of course good ones too!!) with some of the care providers. I remember burning my leg (so badly that my tights were melting) on a baseboard heater at church when I was 4. We were in a prayer circle when it happened. I tried to tell the "teacher" what had happened...she just said "Shut Up-We're Praying!" :
I never had to go back to that group.
Just because it is a church doesn't mean they will treat your kids with respect, unfortunately.