HUNGRY 6 year old!

My husband and I only spend time with our stepdaughter twice a month.  Her mother is a chronic liar, and thus we never know what exactly happens in their home, including what her eating habits are like.  Additionally, my stepdaughter can't articulate her usual schedule very well since she's only 6.  The last couple times she has come, she absolutely will not stop asking for more food.  Honestly, I'm just so tired of hearing her talk about food and having to prepare things for her.  Based on what my husband knows of her mom, we expect that her diet consists mainly of fast food and highly processed food.  She only has access to raw and natural foods in our home and is overeager to eat all she can of what we have in the house, which makes me think her body is craving nutrients.  
My question is two-fold.  Is there anything we can give her 4 days a month that might give her an extra boost of nutrients?  Also, what are some possible reasons for her constant hunger, and what is a healthy way to handle the situation since I want her to understand good eating habits?
Thanks  

[Renee Radenberg]

Her weight is just above the 25th percentile and her height is just below the 50th percentile, which indicated she is normal for her age. Has her weight remained the same for long periods of time? Healthy children have a steady increase of weight since birth. If she is lacking vitamins/minerals, blood work would determine that. If she is not getting enough food during the week, it may be a reason why she is hungry or it could be an attention mechanism. Suggest feeding her healthy foods while she is with you. Hoped that helped you.