It is not that Charles and William have to be careful what they say in front of Harry it's that being in the same room as Harry gives credence to anything Harry decides to say in the future.
@Vespanest I think this is really important - simply being in Harry's company opens up the possibility of interesting headlines; and I think it was @IdaGlossop who pointed out Harry's unpredictability. So far it has included not only personal insults, which they may be ready to forgive, but also rants containing allegations of unconstitutional behaviour.
I genuinely do hope they can find a way forward but suspect it may take a number of years - and perhaps with Harry on the fringes of the family for a while and seen to behave appropriately?