The cheapest way to new beds, is to make your own... just buy needed material and start constructing. Here http://tinyurl.com/bedconstruction you can find any info you need best of luck, and have a nice day
If the children need beds, and you can't afford them, I would try the RedCross, church, or some kind of consignment shop. Do you have "catholic charities"? They may help(with mattress, not frame.) I would NOT recommend picking up beds people leave out for garbage, under any circumstances!!! good luck!!