Hey Dave,
I just want to make sure I am understanding you correctly first. What you ultimately want to be able to do is headphone cue thruogh a pair of headphones and with a seperate pair of headphones listen to the master output? This would allow you to hear the cue/master at the same time through headphones without any audible music being played out loud and disturbing the kids.
If so you can use the Numark DJIO as you currently are by plugging the headphoens in to preview/cue to accomplish that. Then, I don’t know if you have a seperate ipod dock/radio/ any other device that would allow you to plug the rca output (or rca to stereo converter cord) into and then plug a pair of headphones in to silence the speaker and only play through the headphones.
I hope that wasn’t too confusing but for example I had an ipod dock that I could run the output of my controller to which also had a headphone jack on the back that I could plug headphones in to.