It must be made of strong but light material.
It must expand and lock into the lower portion of a door frame (or similar space) so that it acts as a secure and safe barrier for toddlers.
It must collapse into a size suitable to carry around as an easy attachment to an ordinary nappy-bag.
It could be used in all sorts of places... church, friend's places, grandparents... the sorts of places whose occupants have forgotten or had no experience of the wandering yet speedy toddler.
It would bring peace of mind to the frazzled parent and perhaps enable them to have a decent conversation. For once.
Perhaps it has already been invented. Let me know.