What steps should we take for visa extension and bank accounts after getting an O visa for guardianship in Thailand?

You shouldn't be working either if you are on a guardian visa, that's not a visa that allows you to work either inside Thailand or remotely from Thailand. That aside you will each need Thai bank accounts containing 500k baht to extend your 90 days Non-O visa for 12 months. Sponsorship isn't an option

You need to do 2 things:

Get a bank account opened for you, and get a bank account opened for your wife. Each account should be only in the name of the person who will use it for immigration, no joint accounts.

Go to your immigration office and ask them for their list of requirements for an extension based on education for students attending school in Thailand, and for their list of requirements for an extension of a guardian of a foreign student.

Each immigration office has their own requirements, so you need to get the requirements from your specific immigration office. They should be able to give you a sheet of requirements for both types.

You'll also need documents from the children's school to support both your extensions and your children's extensions. The funds for these extensions will need to be in both you and your wife's accounts, as you are each applying for your own extension. So you both need to have your own money in your own Thai account.

The embassy accepts sponsorship, but immigration will not.