Originally Posted by
pseudolus
Leaving on the Thai passport is fine and entering on the Non Thai passport which has more than 6 months, into another country is fine. It would have to be this way though - leaving on the British passports when they last entered on their Thai passport will not work if I remember rightly. The other problem you have is that Thailand, officially at least, does not recognise dual citizenship. So when they try to enter into Thailand on their Brit passport, there is a slight chance that passport control get shitty and ask for the Thai one.
Out of interest, why not pop down to the local amphur and speak to them and explain the problem and see if they can help? Always worth having a go giving them some sob story. Or the old fall back, give the ex wife some cash to sign an affidavit saying shes OK with them having a new passport but can not attend and get it notarised.
If she concerned you'll run off with the kids?