Welcome to HCM,
Your daughter is having according to your history,mucus cyst
of mucosa,in such cyst recurrence is common as it is caused by minor trauma some times duct of saliva is obstructed.
Small one heals spontaneously non surgical is advised in small one that is saline gargles in new one 4 times a day, which drains fluid without surgery,but in large one surgery is the answer and usually with good result.
Please visit a oral surgeon sometimes adjusant salivary gland
needs also to be removed to prevent recurrence also Laser surgery
may be helpful.
Do not worry it will be treated and she will be all right.
Good Luck to you