The Thailand visa waiver is the Thailand visa exemption scheme that allows ordinary passport holders of certain countries, including the UK, the UAE, the USA and more to enter Thailand, visa-free, for no more than a period of 45 days.
because in many cases the rules are very different depending on the passport you hold. It absolutely matters where you come from when Thai immigration is concerned.
I have a fixed account and have for 4 years. There is no issue at all.
In reference to your question I would do this: wait until you do your extension. Do your 90 day and show funds. Once that is done change accounts. I was contemplating doing a similar thing and that was the advice from Jomtien immigration.
You never know what representative you get the day you do your extension and how they treat you. Might have had a bad night and now you’re screwed because it’s not the full time in one set account.
I learned this the hard way. Sent a specific property link to a few agents I was working with and said I want one just like this. No one came close. Eventually my dim witted brain kicked in and I simply clicked the contact us button on the advert and a week later signed my lease.
My experience is a few years ago but I have gone through several times with my Thai wife and a few times I was allowed to go with her and a few times we were told absolutely not. Much like everything here is seems to be subject to the whim of whomever is standing in front of you. Perhaps because you have children you’ll get a favorable decision. Good luck.