You just have to ask when setting up the appt. Special requests usually require some sort of advance notice. Bringing a pair along with you, depending on her size, wouldn't be a bad idea if she agrees. This would more likely be arranged directly with the sp, so trying to set this up with an agency, mp or micro would have problems.
As far as special requests go, this one is very easy to accommodate IMO, but someone who maybe jumps in and out of the shower is possibly going to have some hesitation.