There are lots of noises that happen during a routine take-off and landing that can frighten even adults if they have not been told what is going on. If something more dramatic happened - such as a fire warning, the flight crew would have been very busy and would have had no time to mollify scared children. A scared five year old is quite likely to start crying and or screaming and this would be a major distraction. Loud noise reduce your mental capacity greatly which is why the first action of any emergency drill is for one of the pilots to silence the aural alarms and alert. It is a simple matter of flight safety.

Five years old is too young for an unaccompanied child or children to be in the flight deck for many reasons, in my opinion. That said, BA do not allow this anyway.