On-going Therapy As Argument Re Relocation of Child: What must I prove to stop my 'ex' getting permission from the court to relocate abroad? One of our children is very sensitive and undergoing therapy since our divorce, in which I am closely involved.

You must try and convince the court that relocation would be against their interests, and result in serious and irreversible emotional damage to them, especially the one undergoing therapy, which could not be continued without your ongoing presence.
Relocation From Israel