What are the requirements for extending a Non-O Visa in Thailand for a foreigner with children holding ED visas?

Hi everyone, here is our situation: i am french and have working permit and Visa in place since mid June. Myr girlfriend and I are not married but have a civil union pact (called PACS in french, quite famous but apparently not recognized in Thailand). We have 2 kids registered at school in BKK and they have ED visas. My girlfriend has received a Non O Visa (for taking care of the kids, she will not be working). Valid for 90 days single entry.

We have been told by lawyers that she needs to open a bank account, make a 500 000 THB deposit immediatly and leaves this money for 90 days. When the 90 days Visa comes to expiration, she goes to the Immigration, show this deposit made on bank account and then she will get Visa extension. Is it correct and 100% sure ?

Since she is pregnant and waiting for our 3rd child, we don't want to get into trouble in 3 months. Thank you