A couple days ago, Trine 4: The Nightmare Prince got its first bit of DLC, and it is free on all platforms. "Toby's Dream" sees our protagonists delving into the dreamworld of the Toby the dog, who Frozenbyte fans may recognize from the developer's previous co-op title, Nine Parchments.
This definitely falls in the realm of "just have some silly fun." Your whole objective in the DLC is to help Toby get all the treats that are hidden within his dreams. In order to access this content, you'll have to progress complete the "Prince's Dream" level in the campaign, and then you should see "Toby's Dream" on the level selection map. Sadly, it does not appear you'll actually be able to pet the pup, so... set your expectations appropriately.
Trine 4: The Nightmare Prince is available now on PC, PlayStation 4, Nintendo Switch, and Xbox One. Four player co-op is supported throughout the campaign.