In American culture, eighteen is old enough for someone to go to Hollywood and make her way alone, but the plot, not just dad, thinks of her as an adolescent. Once Grace is out of the house, Dad worries about sex (which the film calls all the "stuff" she might be "getting into"). But while she is at home, it's all about filling the car up with gas when asked and doing your musical number the way you rehearsed it. I can't imagine any teen who wouldn't roll her eyes at this as some representation of the secret life of the American Christian teenager.
Johnny's past apparently contains a few skeleton-filled closets, but they're all kept safely vague. They give him a reason to not want to be a star and—more importantly—to lend weight to his "do as I say, not as I did" concerns for his daughter. (Personally, I thought the backstory seemed more about justifying his controlling behavior than about providing him with actual life experience that might temper his judgmental attitude. But I've always found the whole father-daughter purity ring thing to be more than a little creepy.)
Grace Unplugged is supposed to be a prodigal daughter story. But it is so committed to its PG rating that it can't really let Grace slide into any serious moral decline in her adventure. She must realize that she made a bad decision, of course. But once she does, it is easy enough to correct. A prodigal story is about humbling oneself, but without any genuine debasement in the second act, the third act return plays more like ritual shaming than genuine reconciliation.
The film's script needs the prodigal story framework, however, because it can't make the broad themes of rebellion and obedience concrete. I counted at least five different montage scenes—usually an indication that the story is conceived in terms of a handful of dramatic scenes with little thought given to what happens before or after them. Michalka is pleasant as a performer, but were she as good as the script says Grace is, I don't think the script would have to keep telling us how good she is.
In one scene, dad watches her online video and has some sort of epiphany. He finally accepts that Grace is grown up and has talent. Still, Grace needs to come home because … God wants her to, and running away from God is bad.
Why God wants Grace to return home isn't really explored. It works better if you don't question its assertions and just take them as gospel truth. If you are young and passionate about something (particularly art or music), then that passion is a danger through which the devil will tempt you. So of course God wants you to give it up, or accept the church version of it.