Is it a condition for ongoing charity (sadaqah jariyah) that a Muslim performs it during their lifetime or bequeaths it to be done after their death, or is it permissible for it to be given by their children after their death, even if the deceased was stingy and not keen on giving charity?

All forms of ongoing charity (Sadaqah Jariyah) mentioned in the question are beneficial. Even if a man was stingy during his life and did not care for charities, if his children or others give ongoing charity on his behalf after his death, it will benefit him—God willing—whether that comes from their own personal wealth or from their inheritance from the deceased.