So my oldest she is the pickiest eater I have met. She will only eat : chicken nuggets , turkey (has…

So my oldest she is the pickiest eater I have met. She will only eat : chicken nuggets , turkey (has to be the whole one roasted) , yogurt , peanut butter , chocolate milk, cereal, pizza An some fruits . No veggies , no cheese , no red meats. She won't try anything new either . I have tried everything to just get her to take a bite. Her doctors are saying she is under weight for her age and height . She is very tall , four years old and only weighs 29pounds. I'm at a loss on what to do to help her gain weight . Any ideas ???