If our intended learning outcomes when teaching about borrowing and debt reflect an attempt to protect young people - along the lines that debt may be universal but it can also be very dangerous - then some attempt to integrate this material into KS2-3 will have to be made. Perhaps with creative writing. By KS4, children may already have "credit".
At key stage 4: discuss ethical issues in economics or citizenship or RE or social studies. What human rights do we have - to luxury goods, foreign holidays etc, if the real price is ongoing debt? How moral are the sales ploys that promote borrowing? How far does language affect our view - "debt" is bad; "borrowing" is necessary; "credit" is easy. What are real case studies of debt like? A Citizens Advice Bureau speaker could help with this. Or, again, as a stimulus for creative writing - what sort of world are we in danger of becoming? What sort do we want?