My 'ex' doesn't pay child support, has "dropped out" of mainstream life, and lives on a shoe string in a third world country.I live in Israel with our children.Could a family court here order his parents, who live in Israel and are extremely rich to support their grandchildren, instead of their father ?

Yes, this is possible, as an exception, and a mechanism exists within Israeli law whereby a custodial parent can sue the other parent's parents i.e. the minors' grandparents for financial support for their minor grandchildren if the non-custodial parent does not and cannot pay child support, but this is subject to proving that the grandparents have "spare" income from which they can do so.