Without more information about the schema of BigTable I’m not going to be able to give you a good answer.
If the values in the columns on BigTable will all exist for all the rows then it is probably an OK design. If the rows which have parents in TableA have different columns filled out than the rows whos parent is in TableB then it isn’t such a good design. In this case each parent table wshould have its own child table with only the needed columns included.
Have a table with millions of rows unto itself isn’t a problem.