ANSWER:
It is not permissible at all to take your kids or yourself to such haram parties. You must never do this again.
This is not kufr, however you must repent sincerely to Allah.
You, as a parent must set an example of good moral Islamic conduct for your children and not be the influence that makes them go astray. You will be collecting sins even after you die of what bad and unislamic you taught them and was a way through which they learnt it!