I let my children,who live in the UK with me, visit their dad in Israel during the Easter holidays but he refused to return them at the end of the visit. What can I do?

File Hague Convention child abduction proceedings for their immediate return, in Israel, at the relevant family court. This can be done via the Central Authority in the UK, or you can file directly, at the Israeli family court, via privately retained Israeli counsel, to save time, and co-ordinate with the Central Authorities, in both the UK and Israel.
Abduction To Israel