The WT book "Family Life" (1978) had something to say on this topic:
The latest mention of it is from a 2006 WT which still condones it:
*** fl chap. 10 pp. 131-132 pars. 2-5 The Value of Disciplining in Love ***
"Many child psychologists put a “hands off” sign on children [...] But in his Word, God says: “He who spares the rod hates his son, but he who loves him is diligent to discipline him.” (Pr 13:24, RSV) A few decades ago, particularly in Western nations, books on child training, with their theories of permissiveness, flooded the market. Discipline would inhibit the child and stunt its development, the psychologists said; and as for spanking, just the thought of it was horrifying to them. Their theories collided head on with Jehovah God’s counsel. His Word says you ‘reap what you sow.’ (Galatians 6:7) "
"A spanking may be a lifesaver to a child, for God’s Word says: “Do not hold back discipline from the mere boy. In case you beat him with the rod, he will not die. With the rod you yourself should beat him, that you may deliver his very soul from Sheol [the grave] itself.” Again, “Foolishness is tied up with the heart of a boy; the rod of discipline is what will remove it far from him.” (Proverbs 23:13, 14; 22:15) If parents hold their children’s life interests dear to them, they will not weakly or carelessly let disciplinary action slip from their hands. "
When I was small it was common for kids to be taken out and smacked for not being able to sit still for 2 hours straight. It happened at every meeting, especially the late night mid week ones.*** w06 11/1 p. 5 Reliable Advice for Raising Children ***What the EXPERTS Say . . . What the BIBLE SaysOn Discipline:Dr. James Dobson wrote in The Strong-Willed Child (1978): “Corporal punishment in the hands of a loving parent is a teaching tool by which harmful behavior is inhibited.” On the other hand, in an article adapted from the seventh edition of the popular book Baby and Child Care (1998), Dr. Benjamin Spock said: “Spanking teaches children that the larger, stronger person has the power to get his way, whether or not he is in the right.”With regard to discipline, the Bible states: “The rod and reproof are what give wisdom.” (Proverbs 29:15) However, not all children need physical punishment.
Despite WT still considering this acceptable to the point of being desirable, I've not seen it quite so much recently.
Maybe parents are deciding for themselves that there are better ways, and it's just the old guard in each congregation who sit and silently fume when children are not marched out the back for a good beating.