You are obviously doing everything you can and some parents just don't want to hear it. I would suggest to you a couple options
1. Terminate and hope that the child has better luck elsewhere
2. Keep her because you know she is better off with you than someone else! and maybe do what the PP said and offer the food then clear it when lunch is over (she won't starve herself)
3. You can try pulling all kinds of literature out for the parents and sit down with them for a meeting, be blunt and honest and explain to them your professional concerns, the risk is that they will pull her out but who knows they may actually absorb some of the info and if its brought up again by another carer in the future it may register that they need to do something .
Stay strong and i am sure it will all work out for the best. Just keep her on the same schedule as everyone else and continue to encourage her.