I use to work in retail, and I can believe the kids were on break....and possibly doing some last minute shopping of their own. It is not uncommon at this time of year for employees to use their break to take items off the floor to put on hold in back to buy after their shift...and Kohl's does allow this (my sister is working evenings and weekends as holiday help for some extra holiday cash). However, if they are not on the clock, they are suppose to take off their nametags.
I do think that approaching two people on the floor to complain about the lack of people in registers is not the correct way to go about getting more help on registers. Many of the floor people are not trained or allowed to work register so they can't just go up and help. If the OP really had a complaint about it, it would have been better to approach customer services and let them know or approach someone on the floor and ask if they could inform a manager that more register help was needed.
But even so, that doesn't give the two employees the right to be rude. If you are going to work in a customer service position, you sometimes just need to learn to let comments go. And to go and talk about a customer on the floor with another employee is really not right. I always found that complaining in the back works just as well.
But it does sound like the customer service person made it right for the OP.