Jim W.
Yelp
There appeared to be a number (probably up to 10) of places that offered light meals within a short walk of Giverny.  My family had chosen this one while I was purchasing tickets.  They have an indoor dining area (with one wall open the day we went; it was decent weather).  I think they also had some outdoor seating too.  
Basically you get a tray and pick items out of a buffet-type selection.  They have pre-packaged items, plus some fresh-made sandwiches.  They server a variety of beverages, including some beers and wine in the small plastic bottle variety.  
If you're here at meal-time, your focus is probably either getting into the garden or, having finished your tour, getting back on the road.  I'm guessing the village of Giverny is not really a 'dining destination'.  Still, this was a perfectly acceptable option for a quick bite en route to or from the gardens.