Thx again flanders, but I dont find the right code position to do so.
I find a solution that works for my problem. I add a listener to the master store which adds a datachange event to all childstores.

Code:
listeners: {
                load: {
                    fn: function(store,aRecords)
                    {
                        var i, j, association, childStore,record;
                        for (j = 0; j < aRecords.length; j++)
                        {
                            for (i = 0; i < aRecords[j].associations.length; i++)
                            {
                                record = aRecords[j];
                                association = record.associations.get(i);
                                if (association.type == 'hasMany')
                                {
                                    childStore = eval('record.'+association.name+'()');
                                    childStore.addListener( 'datachanged', function(){record.store.sync();} );
                                }
                            }
                        }
                    }
                }
            },
Mike