As long as the photos do not include sex acts and that you have signed consent from the models with proof they are over 18, you will not be doing anything illegal as far as I know. there is nothing illegal about taking such photos. the illegality comes in where they are published.
I would, however consult an attorney to draw up an agreement for these instances. I would make sure the agreement states that you are doing these photographs for the PERSONAL use of the model. That, upon payment of your fee, you will turn over all proofs and negatives to the model. I would make sure that you keep copies of proof of age attached to these contracts.
Then enjoy the shoots and don't worry. Need someone to work the lights?
