Do I have to have every thing in windstad manor so my kids can move to it?
Solution 1:
At Mixxiphoid's suggestion, reposting Comment as an Answer.
Kids need beds & storage. They can sleep in either the West bedroom on the upper floor of the main hall or in the "Family bedroom" version of the west wing. You will need to have built the beds and specific chests/dressers next to them for the children to move in.