She is still a child in head.
Can we use in head to mean that she still thinks like a child?
In her head, she's still a child.
She's still a child, in her head.
Mentally, she's still a child.
They have different meanings. One means she feels like a child, one means she has the mental/emotional age of a child despite being older.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.