Yes, there is parking at the Old Post Office for the Welcome Reception (https://parkchirp.com/facilities/the-old-post-office-garage/), and there is an option to valet at The Langham. There are plenty of other parking garages near both venues that will be open to the public as well.
The dress code is black tie optional. Feel free to dress up for a fun night out, but no need to rent any formalwear! To give a few specific examples, for men, suits with ties, khakis with a jacket, or tuxedos would be fine, and for women, cocktail or floor length dresses, and slacks or a skirt and a blouse would work great.
Yes, children are welcome at all wedding events. Please let Greg or Becca know if a family member's name is not in the RSVP option, as there are only invitations for those specifically listed in your RSVP.