Just to add to cws's excellent summary that for BA, "next to" can also mean across the aisle or even in front or behind. Thus, if the cabin becomes busy (which can happen quite frequently in WT+), your wife and children could also be assigned e.g. 31F and 32EF for instance. I don't think you say when your trip is and that will determine whether the seat map is likely to get crowded by then or not.

As cws very rightly says 1) only your family can decide whether it is important enough for you that they all sit together that you wish to pay for it or whether you are happy to chance it, and 2) in the latter case, please, don't expect others to serve as an insurance policy for the possible negative consequences of your own decisions by moving if the family end up being separated. other passengers will invariably sit where they do for a reason and it is the seat they will have chosen - they may want to sit together, it is their preferred seat for whatever reason and they will have chosen it either due to status or paying for it. The realistic choices are pay for the certainty of sitting together or wait till OLCI and accept the risk that this entails.