As far as Bridge Plates go. As stated these are used primarily for Persons in Wheel Chairs. Also since most NEC stations have High level platforms if every door had a Bridge Plate it would delay the train in several ways. Opening the doors and placing a BP at each door would take some time, then detraining, and entraining, then pick up the BP's and key the doors shut. Could take 15 minutes.
IMO a Red Cap should assist passengers boarding Children. A good RC will do that, or SCA, CA, or Conductor. Just my 2 Cents on that.