some Immigrations will interpret the police order slightly differently from another, so it is always recommended that as soon as you have arrived in Thailand on your 90-days Non-Imm-O visa, you should visit the Immigration that serves your area and ask for the handout list of requirements to the application for the 1-year extension
then the complete service was 37,000 THB for the first 15 months including a multi re-entry permit, when not having to show own funds. It is posted on their advertisement. Showing own funds it costs 14,000 THB
The maximum age for dependent children in any case is under 20 years old. It applies if your child visits a school in Thailand, both of you, as mother and father of this child, could be on a “sponsor visa” following your child’s Non-Imm-ED visa and ED-extension
and most embassies have discontinued handing out Non-Imm-O "dependent visa". Only SOME Immigrations are willing to issue them, if the main caretaker is on a 1-year Non-Imm-O/A Longstay Visa
I once advised somebody to ask the embassy by Email why they would refuse to accept proofs of a 2200.- Euros income he uploaded for the application to a 90-days Non-Imm-O family visa and instead asked for proof of 400,000 THB in a bank account (despite the income requirement is only 40,000 THB = 1095.- Euro) and they replied to him "We don't reply to emails". . . . so much about any help you can expect from a Thai embassy
like I said, it was like this in the past. Right now, there are not many embassies left who are willing to issue "trailing spouse" Non-Imm-O Visa to married partners under 50 years old. Some embassies issue them but insist that the main caretaker needs to be on a Non-Imm-O/A visa. And your agent was wrong: IF an embassy is willing to issue a "trailing spouse" visa, the first step is the main caretakers applis for his own 90-days Non-Imm-O Visa. As soon as he get it issued, the partner can apply for her/his own Non-Imm-O Visa based on being a dependent. In Thailand, both need to apply for the one-year Extension of Stay, and again the partner needs to apply for his extension of stay based on being a "dependent". There is no "convert the spouse to a dependent visa". What they missed to say is that inside Thailand, the partner converts to a "1-year extended stay permit", not to a "visa"
there are not many embassies who are willing to issue a "dependent" Non-Imm-O Visa. You need to ask the embassy that serves your area. Some embassies will issue it if the main caretaker is on a 1-year Non-Imm-O/A visa