Nested SOQL Query (Parent-Child) filtered by the number of children

I am trying to do query that displays information from a children object only when the parent has more than one children, meaning the child record has siblings (like filtering by the Parent’s Roll-up field), but the relationship is a look-up field rather than master-detail. 

In theory, this query should do the work, but this is not a runnable query:

SELECT FIELDS(ALL) FROM childrenObject WHERE id IN (SELECT parentId, COUNT(id) FROM childrenObject GROUP BY parentId HAVING COUNT(id)>1)

• I tried removing the "SELECT COUNT(id)" from the condition subquery, so the only information gathered is "SELECT parentId", but this doesn't work either.
• Only running the subquery will give me the ParentId and the count, I need other fields from the child object.

I also tried a query from the Parent Object, nesting the children's but I am not able to see how to filter only the parents with more than one record, they are totalised in a column called "childRelationship.totalSize" in the query result, and I am not able to use this column as a filter condition. eg.

SELECT id, (SELECT FIELDS(ALL) FROM childRelationship) FROM parentObject WHERE childRelationship.totalSize > 1 ;

**I am aware fields(all) will need a limit, I just using fields(all) like a example that I need more than just one field returned in the query.