Have her see a good pediatric neurologist, not just the family doctor. The first thing i thought of was Bell's Palsy as well.
Perhaps her face droop is due to Bell's Palsy. I had a friend that had that just during her pregnancy. Once the baby was born, it cleared up but I'm sure she wasn't thrilled with the slightly slurred speech and droopy side of her face.
If she's having a droop on one side and her doctors tell her there's nothing wrong, it might be good to get another opinion.