That’s so sad that the parents don’t know what is appropriate for the children to wear to church or at all. Those sorts of clothing are the kid of things that in my opinion, Our Lady was talking about at Fatima in 1917, the fashions that would come about that would offend Our Lord very much (sins against the 6th and 9th commandments) I know they are only little girls and not yet grown up. But it all starts there, they are already learning how to dress in a sinful way inciting sin in others and also choosing to treat themselves as an object of desire and not the beautiful and valuable daughters of god that they are. They may well be too young to know what they are doing, but the predators aren’t. I’m a retired police constable so I know this for a fact.
CCC 2522. Modesty protects the mystery of persons and their love. It encourages patience and moderation in loving relationships; it requires that the conditions for the definitive giving and commitment of man and woman to one another be fulfilled. Modesty is decency. It inspires ones choice of clothing. It keeps silence or reserve where there is evident risk of unhealthy curiosity. It is discreet.
The parents not being willing to listen is a clear sign of pride, but one battle at a time. Their children are confirmed and at church.