It would be nice to have accurate info to go by. Did the kids go to Walgreens or Jewels? Did Lisa normally leave at 4, or at 6? Going by the info that seems the most logical to me, I still don't think it was planned. If Lisa was at the gym by 6, it seems she would have to leave by 5:55, which doesn't leave much of a window for Craig to get the kids off by themselves and send them off somewhere Lisa would not approve of.
I see it this way. Craig comes home from work. The kids are watching tv or doing homework, maybe even outside. Lisa is getting changed into her workout clothes in the bedroom. Craig walks in on her, she tells him not to come in while she is changing clothes, he says he'll walk in any damn time he pleases, it is his house. Then Lisa pops off with "Not for long." and the fight starts. She tells him it's a good thing he is going to have time off, because he will need it to move after the court kicks his a$$ out of the house, and Craig sees red. He backhands her, and then he holds her down and strangles or suffocates her. (One thing wrong with my theory is she seems to have been working out a lot, which would imply she could put up a fight.) After Craig kills Lisa, he realizes what he has done, and now he has to cover it up. First thing is to get rid of the kids, and then clean up whatever and load the body in the car/truck. All of the preceding is just my humble opinion.