iluvepcot,
I don't know about the webt site, but you certainly would be able to do it through CRO. We didn't need the trundle because we opted for two connecting rooms for our family of five, but I had my 16 year old daughter who is 5'3"/110 lbs lay on the trundle and she was just as long as the trundle and said she would have been perfectly comfortable on it had it been necessary. Try CRO for your reservation, you shouldn't have any problem booking the room. Depending on height, any of your kids shoud do well on the trundle.
It is a beautiful resort...have a great time!