I would approach it as a parent..

something like

thanks guys for letting me know how you feel
your mom and I have been talking about things and I know you are really worried about making your mom happy and are concerned with how she feels about things (you can pretty much guess this is what happened because of the timing of things)

I want you both not to worry about this kind of stuff. We are your parents and we decided when we divorced that your needs would come before ours and you need both of your parents which is why our custody arrange is the way it is.

You are both priorities in our lives and we want you both to know we love you. You don't need to worry about this kind of stuff.

(end of discussion)

thank them for telling you their feelings
validate what they are feeling
assure them you both love them
let them know they are top priorities to both of you (even if you have to choke it out because you might not be sure they are to your ex)
let them know they don't have to worry about it because they aren't adults...they are kids and their job is to go to school


I think their mom is feeding them kool-aid