Thanks to all for your concern; Mrs Fox is fine! Not sure what that lock-jaw thing was all about. I did read that sometimes the female will fill her mouth and take the food to the kits. Perhaps that is what she was doing and she was trying not to smash the kibble in her mouth...but she has eaten at the terrace a few times just fine since then.
Her kits are getting more frisky about coming out during the day and I saw them weaving in and around one of the mustangs. I hope we can get some better pictures of them!