Sounds like she associates lunch at the childminders with puking (did she puke during/over lunch?), and being in a similar situation triggers her retching. In a similar way to how I can't drink lemon squash any more, after puking it up one time it makes me heave.
Or maybe it's attention seeking behaviour- when she was sick perhaps she enjoyed the childminder fussing over her and thinks if it happens again she will get the same sympathetic response?
Or conversely it could be an anxiety thing- if the childminder/other children acted negatively over it then she might be worried about being sick again, and the worry is actually triggering the retching?
In any case, if I was the childminder I would change the routine and situation around lunch. If the children typically wash their hands in the bathroom then sit at the dining table before being served food, I'd try hand washing in the kitchen, before carrying their food through to the drawings table in the living room. So if it is an association, she won't come across whatever it is she associates with being sick.
I would also completely ignore the retching and fussing and as much as possible encourage the other children to do so. Simply give the food, and if she fusses/retches say something like 'oh, you're not hungry? You may get down from the table and read a book/draw/whatever while the others finish'. That way if it's the childminder's/children's reactions that are fueling the behaviour, removing the reaction should remove the behaviour (it could get worse before it gets better though, in an attempt to get the reaction she is used to).
Just out of interest, you said she did it twice at home but no more. Can you remember anything about those two times? How did you deal with it? Was there anything different about those times?