Symbolism, Imagery, Allegory
When the Marquise de Merteuil compares someone to a child, she's not paying a compliment. Nope. It's a common insult for her. She speaks of Madame de Tourvel as a devout prude whose piety condemns her to "eternal childishness." (1.5.3) When Cécile and Danceny are not following the path she would like, she writes to Valmont that it would be "shameful if we should fail to do what we liked with two children!" (1.51.9).
For the Marquise, adulthood has less to do with maturity and responsibility as it does with having control over oneself and others. She writes to Cécile, telling the young woman that she writes like a child, always saying what she thinks, never deceiving (3.105.12). Clearly the Marquise has no children. Oh, and the idea that virtue has value? Yep. Childish. (4.125.2)
So what's with this image? Children, generally, are dependent on adults. The Marquise wishes to be dependent on no one. If she enlists help, as she does with Valmont, it's on her terms. She follows only the rules of her own making. Anything less is childish.