Of course everything will hinge on what you learn from the kids.
Meanwhile, I think you can make this easier by not making it about Mom, but about what you expect from your students and what makes teaching worthwhile. Of course everything you say will be more diplomatic than how I say it here. You can start with Mom:
"I'm happy to start them on guitar. It's a wonderful instrument. I'd suggest they should keep their basses, because maintaining their bass skills will help them a lot as guitarists. However, I hope the kids are interested in making the switch. I'll do everything I can to help them develop their enthusiasm. But I expect my students to develop a level of interest that results in a reasonable amount of progress from one lesson to the next."
Whether or not the kids are interested in guitar will show in their work, perhaps not right way, but certainly after a few months. If they make progress, great. If not, then it's time to apply some "progressive discipline." Remind the kids that you expect progress. It's your studio, your rules. If it's not happening after you've given it your best shot, then talk to Mom and suggest that the kids might fare better under a different teacher.
All of the teachers I know give out a sheet of information explaining how their studios work, and so the expectation of the kids showing an interest and making progress would be something that the parents are aware of from the beginning.