From the press, it appears that the grandparents weren't making much of an effort to help cabin crew secure the child. I have every sympathy for the captain.
Recently on a LHR-GVA flight I watched in horror as my own 3 year old nephew, seated with his parents a couple of rows in front of me, unfastened his belt and started wandering up the aisle when we were only 30 seconds or so from landing. Fortunately his mother was able to restrain him in time - but afterwards she seemed a lot less concerned about the situation than I had been (I had been on the verge of yelling out "Cabin Not Secure!" when she grabbed the child).
A degree in physics is not a prerequisite for parenthood, and it seems many parents are blissfully ignorant of the risks their children are in if they are not properly secured.
BA are much more professional about enforcing seat belt policy than many other airlines, in my experience (on Iberia there are often cabin crew running for their seats seconds before landing and I don't even want to talk about airlines in Latin America or China).