LOL, Indeed memos are lousy which is why it is also drilled into us every year at our SEP refresh training and often mentioned again by the CSD at our briefing at the start of every trip. However sometimes crew think they are being helpful or avoiding an argument by just allowing it which in my opinion is very dangerous and wrong.
You may be interested to know that BA have now authorised the use of the Amsafe child restraint device which if you are unfamiliar with is basically similar to a car seat restraint for toddlers/young children which can be easily fixed to a seat to compliment the main lap belt. The only catch is that it wont work on the First, Club World or World Traveller Plus seat but works fine in World/Euro Traveller, Club Europe.