With the prices you have outlined, I would go with BA in economy. Disregarding the seat selection fees will presumable make it even cheaper. As with CWS above, I really don’t think you need to worry about paying to sit together when travelling as 2 parents + 2 young children. BA will almost certainly make that happen and you can select at T-72 anyway (before most other economy passengers). Ultimately only you know the importance of having the ‘guarantee’ ahead of time that paying for seats gives you.

BA will also give you EC261 protection on both legs, compared with just the inbound on DL.

Transiting in LHR (within the same terminal building in this case, so not the horror story many make LHR out to be) also removes a potential language barrier. If that is something that concerns you.

Have a lovely holiday.