I had finish my whole wolf family tree but I can’t play as a pup anymore and I am wondering how do..

If you go to the main menu and select your wolf there under "Press to change animal", it'll take you to the character screen. Select the wolf and then tap the tree icon in the top left corner that has "Family" by it. It will take you to a screen with a few bars that say "To start a new family select a character that has grown up to level 10 or more".

You will need to tap one of the characters from your original family, and start a new family with them. To do this, simply tap any character icon, it will ask "Are you sure you want to start a new family as ______?" Then tap yes.

Which ever family member you chose, WILL be removed from the original family, but you can still play as them in the new family. Starting a new family will not reset the character's level or take away any of the items you've unlocked.

After doing this, you can find a mate and once the mate reaches the appropriate level, you can make babies again.

ALSO, you can now fill the empty character slot from your original family with a baby, too.