Just reading your thread again and it connected you'd only had Rosie less than 2 months. That is still very much in the adjustment period which makes it highly likely that she is be trying to assert herself in the pack. That said, the problems I had with my first Pyr are not just a Pyr issue. It was first seen (documented? made headlines?) in Springer Spaniels and was called "Rage" in that breed. There is a distinct glazing over of the eyes that you really can't miss once you've seen it and look for it in the next incident. THere is such an element of lack of normal/lack of control that really stands out. That is what caught my attention in your posts but as I said, that is likely due to the adjustment period and her newness in the pack. It is still something to be aware of and keep an eye out for, esp with a dog of unknown background. I do wish you well. It is a horrible position to be caught in - the knowledge that you must protect your child coupled with love for the dog. The behaviorist ought to be able to help you sort through and deal with this situation. We had help with our Pyr as well and it was so reassuring to have someone knowledgeable work through it with us.
Found a couple of links on aggression and rage that might give you some ideas. From one of the links: This problem (rage) has also been reported in American Cocker Spaniels, Bernese Mountain Dogs, Chesapeake Bay Retrievers, Dobermanns, English Bull Terriers, English Springer Spaniels, German Shepherds, Golden Retrievers, Pyrenean Mountain Dogs and St. Bernards. Again, the number of affected animals is very small.
http://www.cockerspanielrage.org.uk/whatisrage.htmhttp://www.vetinfo.com/daggression.htmlhttp://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47