Tips on getting started on a small instance?

Hello. My younger sibling has recently created an account on a small Wafrn instance (we avoided the main one due to them being too young). However they're now facing the problem that I faced back when I briefly joined a small Mastodon instance way back: there's very few posts, and almost nothing is federated; it feels very barren so they're not really using it and spending most of their time on Tumblr (which is what they want to avoid). What can be done to find more people to interact with and follow on smaller instances?