Problems with bull mastiff and pit?

I have a F 7yr old Pit (65lbs) and 2 kids ages 7 and 8. Got a4yr old M American Bull Mastiff (102lbs) from the SPCA today. Took him home and tried out a few commands on him before I had to leave for work. He growled at me a little but I thought maybe it was bc of the new stressful enviorment. I leave for work and leave all the dogs and kids with my mom. She calls me at work 2hrs later to tell me that he has to go. My female sniffed him, he growled, and then lunged at her. My dad took her out of the room and my mom had to practically lay on him to stop him. He didn't go at her and did stop but she broke 2 fingers and possibly her hand in the process. He is a big boy and very docile but has to be the alpha dog...so does my pitty. Also...whenever the kids lay next to him or try to sit with him he gets up and walks away. They can pet him but will mostly ignore them. Is there any way to get the 2 dogs to be friendly or will I need to take him back? Should I be concerned that he may bite one of my kids? I haven't left him alone with them. Any suggestions? Would you take him back? Oh...he was neutered on 12/22 and she has been fixed for a couple yrs now.