One m ore welcome to FT.

This is not about "extending first class" - this is about aircraft substituted for a smaller one with fewer rows and no indication that business class (there is no first on BA European flights) has been extended. It is very frustrating when it happens - which is not unusual as BA, like most European airlines, use different aircraft types (here 319, 320CEO, 320NEO, 321CEO, and 321NEO, each with different capacity) interchangeably depending on how full different flights are and what rotations optimise aircraft use.

Whilst it is possible that the crew will indeed be able to sort things out, I would not take this for granted. With as many as 42 seats lost, the flight will likely be full and other people may be in the same (or worse) situation compared to you, including potentially the person sat next to your partner and son. So things may be easy to fix or not, and worse if your seat happens to be a middle. Note that BA's policy for children is to sit each child next to one adult in the booking so they have done what they commit (and are requested) to do and won't necessarily feel that there is any urgency to do what you would like them to if it is not easy to achieve. In fact, the policy would even consider “next” as across the aisle or row in front or below so if your partner and son are “properly” next to each other, I think it’s likely that on such a destination some people will be worse off and a greater priority to accommodate. (And yes I know, the fact it could have been worth doesn’t make the current situation any more pleasant for you but just trying to explain why some others are likely to get priority over you for possible seat changes).

One thing that noone has mentioned is of course that if the worst comes to the worst, there would of course be no issue with you swapping with your partner if, for instance, you feel that you would be in a better position to support your son during the flight than your partner. Also, I wouldn't start from the assumption that there will be significant turbulences. Most flights are peaceful and hopefully this will be the case here.

Hope you all end up sitting together in the end, but I think it's important to manage expectations and hopefully you'll have a great holiday anyway. And of course, yes, do ask for a refund of your seat booking fee.