I do not recall the small children thing, but a few years ago when the anouncement was made that those with "special needs" should board now we attempted boarding ("we" being myself, spouse and four children age 6 months to 6 years). We were denied boarding and told to wait our turn. Once our row was anounced in we went. With stroller and all the bags needed to keep a couple of kids in diapers clean and entertained it took a while. All sort of people gave us "the look". Not a happy time!
I would like NW to preboard those who would take extra time, including children. This will not speed up boarding, but would make it easier on everyone.
As a note, I fly out of RST. We often have at least 2 passengers in wheelchairs and sometimes a lot more (CRJ/Saab planes). The agents are usually aware of this and start boarding quite early so that the rest of us able bodies can board easily. We always depart on time.