Thanks for your query
Fromage frais are cultured dairy products obtained by fermenting milk. It can have a variety of micro-organisms that are usually healthy. The cream content of fromage frais can vary as well. I would suggest that you wait at least 2-3 months before introducing cow's milk after the previous consumption of fromage frais. It will give a chance for the immunity of the babies to settle down. When you introduce dairy products, watch for development of symptoms of allergy. Keep an epi-pen at hand. In case of a bad allergic reaction
take the kids to nearest emergency room.
Could the twins have allergies? Food allergies
can be tested by a doctor by doing a skin prick. It can also be tested by doing blood tests and checking for immunity.
Since the twins have been exposed to dairy products, they can be checked for allergies by the doctor. It's possible that the symptoms to fromage frais were caused my milk allergy
Family history can matter in case of milk allergy. Milk allergy is also common in younger population and can improve as a kid grows up.
Hope I was of help