Are you building a treehouse for the young ones, then you will need to consider the size of the mattresses and seating, Given that the area you have to work with is confined and limited. Other important points to consider are the depth of the mattress, the colour of the fabric, for instance a waterproof fabric maybe better than the standard cotton coverings.
A quasi treehouse mattress maybe a great idea, thats suits both seating and sleeping, We can certainly double the depth to give you that option.
You may want a thin mattress that you can roll and put away, In the summer months when the grass is dry, you may want to roll out a set of thin mattresses in the garden so the kids can play freely.
Memory foam and foam mattresses are fantastic addition to any outdoor and treehouse living space.