Do parents need to show THB 500,000 in a bank account to extend their child's education visa in Thailand?

The 500K baht deposit is needed for a parent getting a Non-O visa based on supporting a foreign child enrolled in school here on an ED visa <- the guardian visa/extension

As you both already hold Non-B visa/extensions you wouldn't be applying for that "raise a foreign child visa/extension"

AFAIK, there is no actual posted proof of funds that you as a parent are required to show getting your child an ED visa thru a school.

The school will provide you with the documentation, and at most offices you can apply for the in country Non-ED 90 day visa when you have a minimum of 15 days left on the 60 day tourist visa / free entry stamp or a 30 day extension from one of those.. ..

Then once that initial 90 days of the Non-ED runs down you'd get more documentation from the school and apply for a yearly extension