It sounds like at home they don't ask and just go because the toilet is easily accessable and readily available to them at all times. It sounds like it is not at your home. Why not put a couple of potties in easy reach to them so they don't need to ask.Right now they have to because they can't go otherwise and it seems like a pretty easy fix to me.
You make a good point; I always have a potty available when we are playing outdoors and they just go...great idea!
Children are great imitators.
So give them something great to imitate.