Clopin has a big heart for children. They are innocent, and you can shape them to your benefit - they're an asset, if you will. If you treat them well, they'll return the favor once their brains develop. He's willing to become an ally to them, whether they are Outcasts or Parisians. He is someone who will wait outside to trap enemies while the children are inside mansions, stealing gold-ornamented china and silver cutlery. And he'll sing for them at night if they ask him. He also gladly tells them stories with Puppet.
He is not their friend. Only a few children ever made it into his closest circle. Esmeralda being one of them. More often than not, he prefers not to be any child's friend. Not in the sense that children consider one to be. They don't notice that he's taking advantage of them to survive on his own. He is their protector as the King of Thieves, yes, but when it comes down to saving himself, he will not die for a child.
He will run before anyone else does and he knows he'll be the fastest. He can't afford to care about every single child's well-being (whether mentally or otherwise). This includes the Court of Miracles but more so the children on the upside. The ones that come to hear him play in his caravan.
Before he became the King of Thieves at the Court of Miracles, Clopin would frequently help the former King - along with the other men of the group - abduct and hold rich people and their children hostage. Most times, they would let the children go free and keep the adults to extort their families and the church. He's helped hang many a mother and father, sometimes even in front of their children's eyes (with his own Court's children cheering, of course)...
There is no such thing as privacy or ethic at the Court. The kids grow up the same Clopin did and he doesn't care to change that, for it's impossible to uphold in a setting such as the catacombs.
Undoubtedly, Clopin has done cruel, cruel things in the eyes of a child, and he doesn't hide it. He doesn't encourage children to trust him, and he trusts every parent in Paris's streets to guard over their offspring when they approach his caravan, for he never guarantees they will get to return home that night. Sometimes, simply abducting an upper class Parisian child is just a great way to make some more money. And he hasn't a qualm about doing it. It's either kill or be killed. If he cares about kids, the toll of survival is less intense in that moment. He sings on the street to make money to be able to eat not to starve, first and foremost. Not to entertain children. And even though he does have a heart for kids - knowing their innocent (all of them, unless you train them to be little monsters) - he's grown rather cold at heart about the fate of any kid that isn't part of the Court of Miracles.
This opportunistic view of youth is precisely why Clopin draws a sharp line between his own kin and the Parisian elite. While he treats the Court’s children like a long-term investment portfolio (grooming his own tiny soldiers with songs and puppets to build a fierce, survivalist loyalty that eventually turns family over time), the wealthy Parisian kids are nothing more than immediate capital.
He tracks the growth of the Court's youth with the cold eye of a master craftsman inspecting his future tools, but a Parisian child’s tears are merely a useful distraction to pick a pocket or an opportunity for extortion. At the end of the day, he will always treasure his own blood over any French outsider, viewing the children of Paris not as potential allies, but as disposable targets in his endless war for survival.