What word (ie. adjective) and any expression that can be used to describe a person especially a child has an annoying habit of preferring to eat fanciful food (i.e. Sweets, cakes, etc.) instead of regular food (i.e. rice, etc usually eaten as a meal) for a meal. Hope I am understood.
A child who who will eat only a few foods and refuses to eat many foods can be called a picky eater. However, the choices do not always lean towards sweets, and the pickiness can be arbitrary. A picky eater might be willing to eat white rice but not brown rice, or green beans but not peas, or chocolate desserts but not fruit desserts.
If you were to say "She is a picky eater", most speakers (of AmE at least) would understand the phrase to mean that it is difficult to please the child at meal time and that the child might not be getting a balanced diet. Depending on the tone of voice when making that statement, the listener might infer that the parent is annoyed or vexed or exasperated.
Another colloquial expression is
She turns her nose up at everything but sweets.