If there's one thing to move Leslie's heart immediately, it's children trying not to get caught. There currently isn't a definitive sign that this child in particular is being abused, but a combination of Leslie's own experience and having already seen one definitely abused child in this dream means she doesn't assume that a kid is trying to sneak outside a castle barefoot into the snow for general mischief reasons. So she's immediately so invested in this child getting to freedom(?) that she doesn't notice the real Corrin (if she's around) and she ends up trying to move quietly herself.

But there are those footsteps. "No, you're going to get caught. You have to find another way!" she whispers fervantly at the memory that can't hear her.