Tomatoes with grapes instead of vinegar

In recent years, with the advent of our daughter, in our family, a recipe for pickled tomatoes with grapes has taken root. I don't want to give a child with vinegar, this is how the usual marinade is made, but without vinegar, and we add a handful of green grapes to each jar (I mean, not by color, but unripe). Grapes give their acid, there are such jars in my pantry without any problems. Very tasty, and tomatoes and grapes are eaten

Yana, at what stage of its development should grapes be?

Sour, completely immature, you don't have to pick it off from the brush - it is also sour and gives its own taste.