Took me a hot minute to figure it but you gotta get pregnant, keep the child and then select the tree when you feed it enough to have it grow
When playing as a female character: Head out of the city and into the forest. You will find a male wolf there. Walk up to him and let him grab your character, and a scene will occur. Your character will get pregnant. Wait a few days at your character's sleep location (where they will wake up), until your character is very visibility pregnant. There should be another glowing spot near your sleeping spot. when you are pregnant enough, you can give birth by hitting F. It will then prompt you to keep (the heart) or abandon (the X) the new child - click the heart. Then get milk (see my comment below on the source of milk for each character until the milk meter (just above your health) is full. Interact with the baby (hitting F) and let them drink all the milk. Once the child has had enough milk you will be prompted to either abandon the child (the golden eggs) or play as the child (the forest) - click the forest.