My 'ex' left home even before the divorce, to live with another woman. He has not bothered to see the kids for a couple of years now,and doesn't pay child support either. If I leave Israel with them and go back to Europe with the kids, where I come from, could I be ordered back under the Hague Convention?

Possibly, but you might be able to successfully defend proceedings because the court in Europe hearing the case may refuse to order the children back if you can prove that their father was not exercising his 'rights of custody' when you removed them – i.e. did not visit them, provide for them financially in any way etc.