Give the mothers a supply list with the fabric, thread & batting needed and list your labor as $$$ & approximate time. (Give yourself extra time for what ifs. Customer never complains if you deliver early.)
If they buy the supplies you know they are serious. Each of you can sign a copy of the supply list as a contract.
Another thing to consider is the boredom factor of making the very same quilt repeatedly. If you don't like to make the same thing over & over, can you change it enough to satisfy your creative muse and the mother's expectations.
Add to bottom of list any changes mom makes will cause a change in your labor charge.