They had to get naked in the woods at least once a week regardless, so Kate didn't see any reason why they shouldn't let Stiles become familiar with their wolves. It might be necessary at some point in the future for him to recognize them like this and they had limited ability to communicate in this form.
She made a low noise that almost sounded like laughter when Stiles said that Nick wanted attention. Because of course Nick wanted attention. That was his personality with two or four legs.