Help with feral who had kittens in my garage loft

Help!! I'm afraid they will call out of the loft where they were born and be hurt or killed. I want to move them to a big kennel on the floor but I think the feral mom will try and take them back up into the loft! What should I do?? I hear them running and playing behind old boxes but can't see them. Mom is very afraid and even though I've been feeding her I can't get close to her. I have no idea how old kittens are I'm guessing 3-4 wks.

Cover the 3 sides of the kennel with a blanket so it is more appealing to mom.  Make sure there is fresh water and food available all the time.  And have a litter box inside as well.  Will it be big enough for a small cat tree/scratcher for some vertical space?