Early Rider Belter hub maintenance
Fastace is a maker of bicycle components. That looks like a freewheel with one belt drive sprocket instead of 5 or more chain drive sprockets.

See the Terminology Index for a description of the difference between freewheel and freehub/cassette systems.

The belt drive freewheel should be removable with a four tooth removal tool (see the Park Tool video below for tool selection guidance). You'll then need to get inside the freehub to clean and re-lube the bearings and racheting mechanism. I believe that's done using a pin spanner to unscrew the rings with 'KENGMIN fastace' written on it. See the RJ the Bike Guy video below for a idea how the might be accomplished - your freewheel might be a bit different, but it gives you and idea of what to expect.

As an aside, that's an interesting kids bike. Not sure I like the radial lacing of the rear wheel though.