Replying before I catch up today, so I apologize if I'm repeating things already said.
Flights to LHR have crazy high fees. We do our best to avoid them and tend to fly into CDG because it works well from our local airport. There may be better options for them depending where they are going and their departure airport. If we just want to see London I just deal with it and pay the fees. In 2020 we decided to take the train to Paris, stay there for a few days for fun, and then fly home from there. There are still fees, but it's less than half what we were quoted when we looked at booking BA through LRH. Dublin is another option if they are just visiting the UK.
Hotels aren't made for families in general. We struggle every single time because there are 3 of us. I can't imagine what I'd do for 5. I have read (but have no actual experience) that places in the UK like Premier Inns and Holiday Inns have what they call "family rooms" that will accommodate larger parties. I haven't found anything like that at the higher end hotels that I've looked at. DD traveled to Europe last spring break with her college buddies and they found that Air B'nBs worked really well for a large party. Normal hotel rooms in Europe typically allow only 2 people per room.
As for CCards, having one chip and pin would be nice for the London underground. We used Apple Pay which worked on 2 out of our 3 phones (yeah, that was fun) because we didn't have one. Mostly we use the Platinum card, the Gold AMEX, and the CSR when abroad.