My ex-husband has lost his job. Does his duty of child maintenance fall away?

It does not fall away; the duty remains, but the amount may need to be adjusted for a while to his means. Allah says: “whoever's provision is restricted, let him spend from what Allah has given him” (Surah at-Talaq 65:7). Your ex-husband needs to be upfront and make an effort to find work, not go silent. If you have a maintenance order, any reduction must go through the court, not a one-sided decision. Meanwhile, make an emergency budget for the children and check assistance they may qualify for, such as zakat or the Social Welfare Department (JKM). Also discuss when he expects to resume paying in full. See zakat and welfare assistance.