I highly doubt that when the children see that amount of melted cheese and overflowing gratin don't put the spoon in the soup. Then they will neither hate onion nor look like Mafalda again. The original recipe for French soup recommends confit over very low heat the onion and then boil it with the same care with broth for several hours. That way we will get a very tasty soup.
French onion soup
If you like cheese, this onion in French Onion Soup has it as one of the main ingredients.
Image: bostontravel