When it comes to seat swaps, the (not legally binding) rule is if you are the one asking for a swap, you need to offer a seat that is either better or at least equal to the seat you are requesting to swap to. Of course, "better than or equal to" is in the eye of the beholder and itself subjective to each person, but I have to think that in most cases, a C+ aisle is almost always going to be preferred to a standard Main Cabin aisle seat.

In this case, if the mom wanted to sit with her kids (which itself is probably not an unreasonable request), she should have been asking the people a couple rows back in Main Cabin if they would like to move up to C+ so she and her kids could sit together in a row in Main Cabin. And while I'm not one to like confrontation, I would have had no problem suggesting this to the mom, or letting her know I paid for a C+ seat and I'm not moving back to a Main Cabin seat. If she still balked, then get the FA involved who will either sort it out themselves or contact the gate agent. Did the mom know what she was doing by moving her party to the better seats while playing the "I have kids" guilt card? The pessimist in me says probably but I cannot say with 100% certainty if the mom knew what she was doing by suggesting you move to the worse seat of if she was unaware of the difference between C+ and Main Cabin and just happened to have some of her party assigned to C+ and some assigned to main cabin by the GA or the system - all the more reason to say if you're uncomfortable with the situation, to then get the FA involved who will if necessary, also get the GA involved.