I too have heard the story that they had to be strenghtened to accommodate amorous couples, although I don't think that is the cause of the high cost. Quite simply everything you fit to an aircraft costs a ridiculous amount because you have to test it, prove that it is airworthy, get the CAA or equivalent to certify it, change all the maintenance and repair manuals etc etc etc.
As to what they are made of, I don't know, but I suggest that a carbon fibre skin with a honeycomb filling would probably give the lightest solution with the rigidity to inspire confidence in mothers both actual and intending. Fibreglass would be cheaper for the skin but might be bendier and in any case so much of the cost is made up of the design and certification expenses that the price of the component materials is likely to be lost in the noise. The biggest problem with composite materials in this application is going to be the stress at the attachment points such as hinges. You are likely to need to build in doublers to spread the load out into the structure.