@Al***
I honestly don't know what you want as far as answers from the group.
You asked about proof of onward travel, I told you to check the website and see what they require. You said you're applying in Washington DC and they clearly spell out that you DO need proof of onward travel to get a tourist visa from them
Now as far as what documentation the consulate will accept to give a visa to your child you need to be talking to the thai consulate you're applying for the visa at. :/ No one else is gonna be able to give you the answers.
I'm sorry if it seems like we're not helpful but it's the consulate you need to be talking to.