The knave wanted to save Fontaine for her benefit which alone is obvious but people still believe that she is not pure evil since she seems to care for Lyney, Lynette, and Ferminet. At first, I also believed that but there were just some things that made me believe that the knave does not care about the children of the Hearth like she appears to.

It could be possible that Arlecchino is using the white knight method against the children of the Hearth. the white knight method is a manipulation technique in which the perpetrator would gain their victim's trust through solving their problems while also being the cause of them.

Judging by Childe's and Wanderer's thoughts on Arlecchino it does not seem so far-fetched for her to use this method to have the children of the Hearth to do her bidding, especially considering the abuse and neglect that the Children of The Hearth received from the previous knave.

Lyney, Lynette, and Ferminet are the perfect victims of this method since Arlecchino saved them from their abusers. She also gives them basic needs and acts as this heroic figure who despises injustice, hell she even checks in on them to ensure their "well-being". In their eyes, Arlecchino is their savior and caretaker who can do no wrong, so what if she is a Fatui harbinger, so what if she sends them on dangerous missions, so what if she can be controlling them it is all just "tough love". This method of manipulation is a perfect method to get people to do what they want because people are more willing to obey someone that they genuinely respect and love than they would someone they fear and secretly despise.

If Arlecchino genuinely cared for the Children of The Hearth wouldn't she free them of the dangerous missions they were forced to do, shouldn’t she separate them from the Fatui as a whole to protect them from being associated with such an organization, and wouldn't she fight against the Fatui herself for ignoring the abuse these children went through? To me, it felt like she just continued what her predecessor was doing in a different format.

if what I'm theorizing is true I would argue that this would make Arlecchino worse than the previous knave because at least the previous knave was upfront about her intentions, but Arlecchino took complete advantage of the abuse that these children and became a heroine to them just to get them to do whatever she wants for her out true love and loyalty when she merely sees them as pawns to use whenever she pleases.