In chapters 8 to 11 of "The Watsons Go to Birmingham," Mama and Dad's actions in locking up the nursery and using it as a form of punishment become suspicious to the kids and the Watsons.
In chapters 8 to 11 of "The Watsons Go to Birmingham," the kids and the Watson family become suspicious of Mama and Dad's behavior when they start making changes in the house.
The first suspicious thing they notice is that Mama and Dad lock up the nursery, which is a high-tech room that can create any environment the children desire.
This worries the kids because they have been spending too much time in the nursery, and now their parents want to restrict their access to it. Another suspicious thing is that Dad threatens to close up the nursery and take away the kids' privileges unless they do their homework.
This shows that Mama and Dad are trying to control the children's behavior by using the nursery as a form of punishment. The kids are confused and concerned about their parents' actions because they have become reliant on the nursery and see it as their mother and father.