I’m editing this old post because there was an error in the original comment.
function dffs_PreSaveAction(){
var hasChildren = false;
if(spjs.vLookup.dataObj["The_name_or_your_vLookupField"] !== undefined && spjs.vLookup.dataObj["The_name_or_your_vLookupField"][spjs.dffs.data.thisItemID] !== undefined && spjs.vLookup.dataObj["The_name_or_your_vLookupField"][spjs.dffs.data.thisItemID].count > 0){
hasChildren = true;
}
if(!hasChildren){
alert("You must add a child item before you can save this item");
return false;
}
return true;
}
Alexander