The trundle beds are all in Alligator Bayou.
There is no charge for the trundle beds, but there is a $15 charge for each adult past the first two for each night they are in the room.
Having a family of 5 does NOT guarentee Alligator Bayou with a trundle.
They can put you in a Mansions room and provide a roll away bed.
Roll away beds can be requested for the Mansions, and THEY cost I believe $15 a night if your party isn't a party of 5 which gives you the bed as part of your ressie.
Parties of less than 5 are assigned to the AB section if they are available, but Disney does try to assign them to people with 5 in the party.