If you book "preferred" you will be guaranteed to get one. Otherwise I would think you're taking your chances. If it's that important than I would pay the extra $30 a night for preferred. I booked a preferred room for that exact reason, knowing that my DS6 would be in HEAVEN in that little bed.
On that note, I hope the adult that will be sleeping on the trundle bed knows that it's about the size of a toddler bed, smaller than a regular twin.
~Cari~