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! 

I was wondering if I take my kids to a party where they play music with some kufr lyrics. I will not let them enjoy and dance and explain not to enjoy in their heads. But if they end up enjoying it in their heads will I become kaafir because I shouldn’t have taken them to the party in the first place.