It sounds to me that the plan is for the attic to be converted/utilised into a playroom albeit without sky lights or dormers and perhaps said staircase might be going into the kids bedroom? Am I close?
Just be advised those space savers entice people to come down facing forward which is very...
I’m not sure where sells them but they are very steep and often only used as a more permanent access to lofts or indeed converted lofts that haven’t got PP or BR. Good luck