What is the amount of zakat due on my money and that of my 12-year-old son, and is it permissible to give a portion of this zakat to the maid, the building's doorman, and the sanitation workers?

If the responsible employee bases his statement on the fatwas of scholars, then there is no harm in relying on what he informed you, as he may have mentioned the amount of zakat to you after deducting the assets. You can verify this by contacting the scholars from whom he quoted.

Regarding paying zakat to the guard and the maid, the categories for zakat expenditure are limited to the eight descriptions mentioned in Surah At-Tawbah, verse 60. So, if these two workers are deserving due to their poverty and need for support, then there is no harm in giving them from the zakat.