Jacee858 I am wondering whether this is separation anxiety. It doesn't always hit straight away. I had much the same experience except that other Grandparents don't help out, so it was just with me that GD didn't eat.
It was infuriating as I really wanted to make sure that GD had good diet on my days to help out (It must be hard for DD to produce a good quality meal after a day at work)
I think you are doing all the right things. You could try delaying evening meal until Mum/Dad arrives and then eating as an extended family to try and break the cycle - or don't bother doing an evening meal and instead make a meal for Mum/Dad to take home and feed.
My fussy autistic son went through a very worrying anorexic phase at 2 when I was heavily pregnant. I remember feeling quite despairing. We used to go to a self-service cafe with all the food on display. We would arrive and discuss that everyone was eating, then queue up and look at the food and the food that other people chose "Mmm that looks yummy...", make our choices and then sit down to eat, surrounded by other people doing the same. I think that was the first and last time he ever ate rhubarb crumble and custard LOL.
Tried it with GD at Ikea cafe. Ended up wearing horseballs. Not a success. [wine makes it better] emoticon