Laura,
I wish I had a magic wand a could make everything all right, I don't. Soooo...take a deep breath and relax. Have a Hug! Here is my 2 cents worth.
You are being a loving, caring respectful daughter. Your father, I'm certain, is proud of the help and support you have given and are yet to give.
Your 8 year old daughter is the same age as my Kaitlynn. I am no expert on children but my opinion as to why she is acting out would be this. She has recognized things are not normal, she has recognized that you are upset and spending time going with your father, her normal is changing, your attention to her is changing, and she is scared and frightened. She probably doesn't understand cancer and the process of treatment and recovery.
I would suggest that you set aside some time for just you and her. Tell her you love her, answer her questions without going into more detail than you need to, and try to reassure her that things will get better. I know this is a difficult task but you need to show her that this situation is only temporary for now.
Try and have Pop-Pop spend some time with her and see if he can help her understand. He may be scared and not want to dicuss the situation but, if you explain to him the affect that the whole situation is having on her maybe he can help. His spending time with your daughter may help him as well. If I was in the same situation with Kaitlynn this is what I would try and do.
You should talk to your step-mom. Whether she likes it or not she will be involved in the process. Tell her you need her help and support and that you will give the same in return. She needs to be an active player. She also needs to understand that you are a mother who cannot drop everything and ignore your children.
Recovery:
Your dad is going to experience a lot of things. I am sure his doctors will make sure he has minimal pain and discomfort. You should expect that his jaw will be sore, his neck and shoulder will be sore, and his hip where the bone graft is taken from will be sore as well. How long? How much pain? This is different in every patient. Make sure you talk to the doctors about pain management. Ask as well about how much change in appearance he will have. Explain that you would like to prepare your children for this.
You are a strong and caring person but, realize that you cannot be all things to all people. Do the best you can and don't be afraid to ask for advice, help and information.

I hope this is a little helpful.
My thoughts and prayers are with you and your family. E-mail or Pm me anytime if you want.

Cheers,

Mike