Hibernate custom child SET Data

I have 3 tables say A,B and C. Tables B and C references parent table A.

Now when I execute "from A" its gets me SET object of table B and C.

Now I want to change the way data for table B and C is fetched. In my scenario I don't want to pick rows from table B where certain column is blank. And I dont want to pick records from table C where certain column value is less than 50.

What should be the best way of making this work ?