Winnie is waiting for midnight, because that is when she knows that she will be able to sneak out of the house. Winnie needs to sneak out of the house that night in order to help break Mae out of prison.
Miles's plan to break Mae out of jail is fairly straightforward. He plans to remove the entire barred window from its frame. That will give Mae enough room to climb out through the enlarged hole. He is able to do this with a fair amount of confidence because he works as a carpenter when he is away from the Tuck family. The real problem is that the constable is watching Mae fairly closely, so he will notice Mae's absence quite quickly. That won't give the Tuck family much time to escape.
"Miles's got a plan, but I don't see how it can work," said Jesse, speaking quickly, his voice almost a whisper. "He knows a lot about carpentering. He says he can take Ma's window frame right straight out of the wall, bars and all, and she can climb through. We're going to try it tonight when it gets dark. Only trouble is, that constable keeps watching her every minute, he's so durned proud of having a prisoner in that new jail of his. We been down to see her. She's all right. But even if she can climb through the window, he'll come after her soon's he sees she's gone. Seems to me he'll notice right off."
When Winnie hears this problem, she volunteers to help. After Mae climbs out of the window, Winnie will climb in. She will lie in the bed with a blanket over her, and the constable will think that Winnie is Mae and that nothing is wrong.
"I can help! When your mother climbs out the window, I'll climb in and take her place. I can wrap myself up in her blanket, and when the constable looks in, he won't be able to tell the difference."