They are "roaming" characters - so they don't do a stationary meet & greet the way other characters do.
In the morning, they can often be seen by/near the carousel
They hang out periodically at the back of the castle, or in the vicinity of Castle Couture (we saw them across from the Cinderella fountain a few times).
And they roam around during the Halloween party, that's where I was finally able to catch them!
You kind of have to engage them and play along with them to get them to stop and take photos, but it's super fun trying!