That is really a tough situation.
I would definitely have Rosie checked at the vet, and by a behaviorist, to rule out any problems. If she gets a clean bill from both, keep her, but don't leave the 3 year old alone with her.

Since she has been a member of the family for so long, and such a great dog thus far, excluding this incident, it may very well have been a one time thing. She may have been provoked. Unfortunately, we will never know.

Please let us know the outcome of the situation.