100% music lessons. They aren't cheap.
Also sport support. Ds was into athletics for years which is probably one of the cheapest sports to do, but it was still a stretch to pay club fees, club kit, trainers, spikes, EA membership fees, competition fees, travel to meets. I'll be honest, when he had a developmental injury I also paid for private physio, though with hindsight I would have gone via the NHS first and got the private physio after he'd seen the consultant.
I also got a tutor twice for ds. Once when he had trouble stepping up a level in maths, and for a few sessions when he needed to take a particular subject exam for his degree (the TMUA).
I would talk it through with your parents. Clearly private school fees aren't needed in your case. But they could invest the equivalent of a year's fees for each child for the future, and give you the equivalent of a term's fees each in cash to put in an interest-earning easy access account. That term's fees would pay for all the costs I've described above and lots more - school trips, new experiences like theatre, dance, travel.