I have not found Kevin to be that high maintenance of a child compared to his other siblings. I remember the episode with Kevin's family therapy session and Rebecca straight up admitted that she needed Kevin to be alright because her hands were full with Randall, and then Kate and Jack had their own thing. Which as that kid in my family, it sucked as a child and it hurt like hell when you realize what's up with Mom and dad. But, I also do understand Rebecca's point of view now that I am an adult. She has so much on her plate with Jack, Randall, Kate, cooking, cleaning, her mother, maybe her father and his health, Jack's dad's decline etc etc. As an adult I now understand the emotional labor Rebecca (and moms everywhere) do on a daiy basis. If Rebecca (and my mother) needed one kid to be okay so she could tackle the next task, I get that now.
We haven't seen much of toddler Kevin so I don't know if that was true back then as well. He could have been a terror or a good kid. We shall see next episode I assume.