I see him like a child because he's sorry that he got caught, not really that he did something wrong. He can't shoulder the blame on himself as he ought to, without throwing in a 'She started it!' as he does when he says, of Eliza, "I do not mean to justify myself, but at the same time cannot leave you to suppose that I have nothing to urge, -- that because she was injured she was irreproachable; and because I was a libertine, she must be a saint. If the violence of her passions, the weakness of her understanding -- I do not mean, however, to defend myself." If he doesn't mean to defend himself, then why say that about her?

And most of all, he's like a child because it's like he had a favourite toy taken away, one that he never took care of properly when he had it, but now that he can't have it anymore, that's what he wants most of all--that's how it was with Marianne. He didn't value her while he had her, and now that she is lost to him forever, he realizes he would rather of had her than what he gave her up to get.