Give Willow a lot of gifts, and talk to her until you get 4 hearts.
Then, give her a diamond. You can buy one from the cafe for 200 coins(use the code "snowfall" if you can't afford it).
Then, keep giving Willow gifts until you get 4 rings.
After getting 4 rings, keep sleeping in your den until she asks you if you would like kits.
Answer "yes," "y" or something like that.
After answering the question, wait until you get called to the nursery and meet Willow and her kits. It takes time until you get called to the nursery, so be patient.