Dentist here...I think all offices around the country have been mandated by their state governments to stop all elective care and can only treat emergencies (meaning, pain and swelling etc). While your dentist and orthodontist's assessment is spot on regarding the need to remove your child's teeth in order to avoid more complex orthodontic treatment later, your child isn't in any pain or distress and the extractions are not essential to alleviate any acute symptoms. Meaning they can be delayed with little recourse in his immediate well being. Bare in mind a lot of dentist's and especially their staff (assistants) are very unnerved right now regarding this virus because it is easily transmitted through aerosol. Guess which profession holds the biggest occupational risk to contract COVID-19? Dentists essentially can be working in a COVID-19 minefield with every patient they treat without knowing it.
On that note, dentist's are now concerned about the long term ramifications that COVID-19 can potentially have on their dental practices from multiple standpoints including the safety of themselves and their staff and patients as well as future impacts on profitability. My point is this Coronavirus has major trickle down affects that are not only inconveniencing every day patients such as yourself and your child (and rightfully so, I fully understand and would be frustrated too) but also everyone else involved in the care of your child. No one, including theme parks and Disney junkies, are going to go unscathed.