My ex-husband refuses to pay child maintenance unless I let him take the children overnight. Is he right?

No; child maintenance and visitation rights are two separate matters, and neither may be held hostage for the other. Children still need to eat and go to school whatever the adults' dispute. At the same time, ask yourself honestly whether his request is reasonable. Children are also entitled to quality time with their father. If you have a real concern about overnight stays, state it clearly. Suggest an alternative, for example daytime visits first. If there is no agreement, ask the Syariah Court to set the visiting schedule and maintenance (nafkah) in writing. See enforcing maintenance.