Just wanted to provide some context and perhaps some peace of mind for any new child passports. I submitted two together that was done together at the local library.
4/7 5PM library appt for passport application for both kids <16yo
4/14 status updates online to application received and processing
4/15 checks are cashed
4/29 status updates online to supporting documents mailed via USPS first class shipping
5/1 received both passports in hand via mail
3 weeks overall turnaround time and I suspect if you make an appointment earlier in the morning directly at USPS, the turnaround time would be faster by 1-2 days since local library would mail the applications the next day which would still need to reach the local post office.
Personally, I was worried they would reject the photos submitted since my children's head didn't have much clearance with the top of the photo (a sliver of gap between top of the head/hair and the top of the photo) but they still took both pictures. Seems like they're fairly lenient.
Hope this helps everyone!