Nickelodeon, the kids’ cable network owned by Viacom, has launched an app on Roku players and TV models in the U.S. — but most of the content in the service is available only to pay-TV subscribers.

The Nick app on Roku offers access to full-length episodes to consumers who subscribe to participating pay-TV providers, as well as shortform videos from the network’s popular live-action and animated series. Those include “Teenage Mutant Ninja Turtles,” “SpongeBob SquarePants,” “Henry Danger,” “Bella and the Bulldogs” and “Harvey Beaks.”

In the Roku app, users can access new episodes and extra Nick-themed content every week by signing in with credentials from participating “TV Everywhere” partners. The app is listed under “Kids & Family” in the Roku Channel Store.

According to Viacom, Nick mobile apps on Apple’s iTunes Store, Google Play, Amazon and Microsoft Xbox 360 have already hit 16 million downloads globally.

Separately, Nickelodeon earlier this year launched Noggin, a $5.99-per-month mobile-subscription service for preschoolers, which includes access to episodes of “Blue’s Clues,” “Little Bear” and other exclusive content.

Nick’s ratings have fallen dramatically in 2015, dropping 34% in the first quarter of 2015. In April, Viacom announced a $785 million writedown, tied to lower ratings, which the company estimated would save it $350 million annually.