My daughter has the same thing
My daughter was diagnosed with it when she was born. She was born with a large cyst on the right side of her neck. She outgrew that, she's 2 now, until about a week ago, a new large one appeared overnight on the left side, on her cheek and neck. It sucks. I'm afraid she will be disfigured. That scares me about it attaching to the bone.
I'm in the process of getting a referral to go to the childrens hospital in Cincinnatti, and see what they say.
I feel for you.