If they had all owned it individually, it could probably go on Schedule E.  But the LLC owns it, so the multi-member LLC needs to file a Partnership return.

One correction ... the child would need to file a tax return is gross income is over $1100-ish.  Because it is a Partnership on the K-1, it will end up being the "net".  But if it had been on Schedule E, it would likely be the gross amount before deductions.