A child can dislike a parent's new partner for reasons that have little to do with the new partner's character.
They may fear replacement.
They may hope their parents reunite.
They may resent losing private time.
They may dislike new rules.
They may simply need longer.
Their feelings matter.
They should not be given responsibility for deciding the adult relationship.
Start with the child's loss
Even when a separation was necessary, a child may have lost: