Bottom line here is that this was a self-inflicted wound.
Travel insurance is the best way to hedge against this risk. It is certain and easy. OP would have simply cancelled the tickets, received a cash refund and then purchased new tickets at some point in the future. This insurance is typically very low-cost for single trips.
OP simply found a more sympathetic reviewer. The threat of action in the UK would not move any carrier. The terms in the KL COC and fare rules are fairly consistent across the industry and the fallacy here is that anybody is "forced" to do anything. These restricted tickets are all about consumer choice. One trades flexibility for price. While this should by no means be taken as a lack of sympathy for OP's child's sore throat, that is entirely forseeable and routine.
OP ought to be grateful rather than gloating. If KL reads this, it will likely instruct its agents to deal with less sympathy.