My husband, daughter and I are all planning to live in Thailand for one year. I already have documents to apply for a volunteer O non-immigrant visa. My daughter plans to go to school there and get a letter from the school to obtain an ED visa. My husband plans to apply for an O visa as her caretaker. Does it matter for the caretaker O visa that another parent is in the country on a different type of visa?

No.

Ones the child has an extension of stay, he can get the extension of stay too, but must show at least 500,000 in a Thai bankaccount. First extension for at least 30 days, all next exetnsions for at least 3 months!

You yourself will need a work permit, voluntering without one is illegal and you will also not get an extension of stay wthout one. Make sure the organisation will help you applying for the WP.