Admob with designed for families not showing ads

I want to add my app to "Designed for families", but for mixed audience, so as official documentation said I change my ads request like this:

Bundle extras = new Bundle();
extras.putBoolean("is_designed_for_families", true);
AdRequest adRequest = new AdRequest.Builder()
            .addTestDevice(AdRequest.DEVICE_ID_EMULATOR)
            .addNetworkExtrasBundle(AdMobAdapter.class, extras)
            .tagForChildDirectedTreatment(true)
            .build();

The problem is that when I put the "is_designed_for_families" extra, no ads is displayed.

W/Ads: Failed to load ad: 3

Update: I know that error 3 means that admob doesn't have an ad to show at the moment, but I have set "test device" so it should show test ads.

I haven't marked as "Designed for families" in Google Play yet, because I want to check that the ads are displaying correctly.

Any ideas about what is happening?