Encouraging the worlds fussiest eater

Hi, My granddaughter is visiting and I can't find any foods she will eat. She is so skinny her ribs stick out. The few foods she will eat are avacados, baby carrots with goddess dressing, an occaisional apple, rice, beans, macaroni and cheese, and sometimes fish sticks or chicken bites (but they may be untouched). This is really it. She more often says, no, thank you. Her parents allow her to eat by herself watching a movie, so there is little encouragement regularly. Any tips? I have read a page here of "tricks" to get kids to eat...she is kindergarten age, by the way.