You could try something like this:
Model.find_by_sql("select * from models where parent_id in (select id from parents where is_allowed='true') and ... ")
Just replace the ...
with your original SQL query.
Edit
Based on your comment, I think this line is more what you need:
Model.find_by_sql("select * from models where parent_id in (select id from users where is_allowed='true')")