Keep your eye on current events. I heard they might do away with conscription in 70. Having your daughters already registered is probably helpful. There was form where they ask about the family and I feel having my sister there to present her ID made the clerks more confident.
So I got it. I seemed to have lucked out as they skipped the interview. Possibly because one of the seniors in the office had worked late the day before and was going to come in late to make up for it. So that division of the office seemed a bit understaffed. Probably would have been a 20min interview as we were told before hand that they only did this for 1 person a day by appointment because it takes 3 hours. So they schedule you first thing as soon as the office opens and we left with ID in hand 20mins before the 3hr mark.
I can offer some observation/advice from my limited experience and speaking with friends and relatives about it...
From relatives that were born, raised, and work in thailand this was a formality. This is my right in their opinion. One cousin even taught me how to politely ask (incase) "what official reason are you denying me? Both my parents are thai as am I. All the documents you asked for are here. What more do you need"
The main thing the government is concerned with here is that the person looking to be registered is the person they claim to be. The fear is illegal immigrant trying to steal a thai's identity. And that is especially a concern with thai's (like myself) who aren't in thailand much. As long as the clerk believes that you're you there shouldn't be much hurdles or grueling interrogation.
With that said you should definitely respect the process. Both parents should be there. You should have all original documents. If your thai children were born abroad you should have taken the local birth certificate to your consulate/embassy and gotten a thai birth certificate. I know of someone that was denied twice before being confirmed and from the story I think it was because she only appeared with her witness and no parents.