C.H.
You don't have to name a relative.
It is necessary of course to discuss this with the person you naem and see if they are up for it. In my mind, you should have enough life insurance that would go to the kids' trust and for their being raised. Get it before you guys get any health or weight problems. Term insurance is cheaper and can go out 20 years.
It only makes sense to then discuss it with his mother on why. If she knows all the reasons you think your friend would be great and that they have accepted the responsibility, and that they know the importance of keeping her as a close grandmother, she may be relieved that she won't be trying to raise a teenager at the age of 60-70.
See if you can get a free consult with an attorney to find out what they offer in protection over and above what a form will would do. I presume it could close loopholes that his mom may want to challenge.
Don't forget what to do if those friends change their minds for any reason (like their health) or if one of them dies or whatever. Always look three steps ahead for potential problems.