I think she did have a plan. Not a well thought out one because I don't think she ever thought she'd have to really explain herself to anyone, but a just in case sort of plan to cover herself. I think she told Kyron to meet her out at the truck. I think it's possible she had a container that wouldn't look out of place in the back of that truck, like an ice chest. I think she possibly sedated baby K so baby K wouldn't a problem or get in Terri's way or seeing anything that was about to happen. Kyron probably thought baby K was just sleeping when he got into the car, and then Terri drove somewhere and killed him, and then put his body in the container. She either tried to get rid of it, or kept it with her and then did things like go to the store to give herself an alibi. After the store, in either case, I think she actually had a freak out moment, worrying about getting into trouble for what she did. I think she drove around a bit, and I think she at some point called Dede and told her she needed Dede right now or it was time for them to meet like they had planned to do.
I think she met Dede to either give Dede the container for Dede to get rid of or to tell Dede to go move it because she was too nervous about where she had dumped it, and then while Dede did that, she drove off, changed clothes, and then went to the gym to alibi herself even more. It's possible she lied to Dede about the contents of the container, or being good friends, she told her the truth and Dede decided to help her. Or maybe she had drugged Kyron, and he had an allergic reaction to whatever she drugged him with and died, and Dede helped her get rid of him since their plan of just holding on to him for a few days to scare Kaine had backfired. It may have started out as a plan to just scare Kaine and eventually return Kyron, but then he died and that changed everything. Maybe Dede was meeting her to get a live Kyron, but when she got there, he was dead instead and she helped Terri cover it up. There are many scenarios here, but Dede either met her as a concerned friend and became an unwitting accomplice, or met her as a willing accomplice and helped her carry out a plan.
Then Dede met her at the gym later to tell her the container was gone and no one would find it, or that the situation was taken care of, and then they went on with their day thinking the police were going focus on perverts casing the school and not two housewives kidnapping, killing, and getting rid of Kyron. Or Terri believed she wouldn't be caught at all, and maybe even had a shower to wash Kyron off of her hands without telling Dede what happened, and felt sure the police would focus on a pedophile and not her. I believe Terri was absolutely shocked that things focused on her, and she just couldn't come up with a good excuse to cover that 90 minute time period other than baby K not feeling well or having an earache. She really didn't think she'd had to cover herself at all, and would get sympathy and attention for a missing child. Dede was either willing or unwitting participant in this whole thing, but I just can't decide which yet.
That's my theory on what I think happened. I don't know if that's what actually happened, but ever since I heard about that 90 minute drive, this theory has made a whole lot of sense to me. I know there's probably holes I haven't thought of, but I do think this is what might have happened.