I would only use plywood when the bed is manufactured with supports that run the full width of the bed - from side to side. This is commonly used in the popular log bunk beds. 

If the bed supports just consist of blocks screwed to the side rails (cleats) then you should use a bunkie board and safety wires.  Plywood is a laminate so it does not have any inbuilt protection against sagging that will eventually cause you problems.

There is very little price advantage between a sheet of plywood and purchasing a bunkie board when you take into consideration  time spent in purchasing, transporting and cutting to size.  However, bunkie boards in extra long twin and queen sizes can be difficult to find.  Plywood may also emit gases from the glue which can be of concern.

Bunkie Boards are sold by most mattress manufacturers.   In my time I have sold hundreds of them and never heard of an accident arising from the actual board breaking.  However, I have never come across an actual test result.  From a practical point of view, they seem to hold up well with the weight of both an adult and a child at story time. So if you are physically able to climb up to the top bunk, you are almost certainly O,K,!

What About Slats?

Although a common form of support, I don't like slats because the don't provide a flat surface for the mattress which can result in poor support for the occupant and reduce the life of the mattress.

What About Making My Own Bunkie Board?

If you follow the construction ideas above, you probably can.  However, buying everything you need, wooden battens, hardboard and covering material on a "one off" basis would almost certainly end up costing you nearly as much in just materials, let alone your time, as it would to buy one new from a mattress store or on-line at the Shopping Warehouse (search on bunkie).

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