How to exclude non-distinct rows in a query
You could use GROUP BY, HAVING and a Common Table Expression (CTE)  to obtain the data.
The GROUP BY and HAVING provides all those PbMIDs that only impacted a single company. If you need PbMIDs that impacted n companies you could change the HAVING to HAVING =n  where n is the required number of companies.
SELECT
    PbMID
FROM
    ChildTable
GROUP BY PbMId
HAVING COUNT(Company) =1; 

This can then be combined with a CTE to produce the final query below.
WITH CTE_SingleInstance (PbMIDSinglInstance)
AS
(
    SELECT
        PbMID
    FROM
        ChildTable
    GROUP BY PbMId
    HAVING COUNT(Company) =1    
)
SELECT
    ChildTable.ID,
    ChildTable.PbMId ,
    ChildTable.Company
FROM
    ChildTable CT
JOIN
    CTE_SingleInstance  CTES
ON
    CT.PbMId  = CTES.PbMIDSinglInstance;