We had this happen with our last FD. The second I wasn't watching she would attack our son. I quickly learned I just couldn't leave them alone together for even a minute. She was 2, he was 3. It was tough even then and she started going after him right in front of us. We (and her therapists) attributed it to her being a single child with a mom that was obsessed with spending every second with her (no exaggeration, mother and child were extremely co-dependent on each other). It was hard for her to adjust to the world not revolving around her every second of the day despite our best efforts to make the transition as easy as possible.
We treated the attacks the same ways Sam-N-Tony's mentioned for as long as we could but in the end, we ended up having her moved because the distress she was causing our son was beginning to be too much for him
