1931 might be poor recollection, but there is a document dated from around that time entitled "Maybury Report - Traffic Signs" in the University of Warwick library. It contains designs for the then-new warning, regulatory and directional signs, and details for signals and the "Llewellyn-Smith" typeface, and details on road markings as well.
Thanks for setting me straight. As a matter of fact I have also seen the 1933 report, but not in any recent year, and I last examined it before I fully appreciated the significance of Maybury having been its chairman. It hardly helps that the copy which is nearest me is in the Bodleian library, so I can't simply duplicate it with a digital camera and load it onto an external U.S.B. hard disk for easy reference. When I finally finish my dissertation and have more leisure, one of my first projects is to go to the National Archives and look up the 1933 Committee documentation.
Incidentally, the "children" plate wasn't mentioned, but then that doesn't mean it wasn't added before the report's findings were implemented (in the same way barred circles disappeared between the Worboys report and the 1964 (?) TSRGD.
It is also possible, and in fact very likely, that the "CHILDREN" plate was added by Ministerial authorization between 1933 and issuance of the next set of Regulations, in 1950. According to the D.F.T. history, the 1950 Regulations were issued in a hurry because a judge had chastised the Ministry for failing to keep traffic sign legislation in a single, consolidated document--in the seventeen years intervening, sign authorizations had been issued on little pieces of paper which practitioners had to collect in order to have complete knowledge of which signs were legal and which were not.