I would talk with the supervisors directly and tell them that you are just exploring options for your future. Tell them that you aren't necessarily looking to leave, but that as a responsible and ambitious person, you are always looking out for new opportunities to improve your options to provide for yourself and family. Reinforce that you have included them on your reference list because you believe that they are people that you can trust to honestly support you and provide an honest and unbiased reference for you.