create trigger UPDATE_BV
on [dbo].[Registrations] 
for update as
if update(Child_Enrolled)
begin
     update [dbo].[Registrations] 
           set Child_Enrolled = '100'
           where Adult_Session_Enrolled = '1000'
     update [dbo].[Registrations] 
           set Child_Enrolled = '200'
           where Adult_Session_Enrolled = '2000'
     update [dbo].[Registrations] 
           set Child_Enrolled = '300'
           where Adult_Session_Enrolled = '3000'
end

update U
           set Child_Enrolled = CASE I.Adult_Session_Enrolled 
                                     WHEN '1000' THEN '100'
                                     WHEN '2000' THEN '200'
                                     WHEN '3000' THEN '300'
                                     ELSE Child_Enrolled -- If something goes wrong! leave the original value there
                                     END
FROM [dbo].[Registrations] AS U
    JOIN Inserted AS I
        ON I.MyID = U.MyID
    JOIN Deleted AS D
        ON D.MyID = U.MyID
WHERE COALESCE(D.Adult_Session_Enrolled, '') <> COALESCE(I.Adult_Session_Enrolled, '') -- Column has changed
      AND I.Adult_Session_Enrolled IN ('1000', '2000', '3000')