YABU because, children or not, you are no longer married.
YANBU because you know him, and well... it's seems rushed.
I suppose it's easy to say let him make his own mistakes, and what else can you do? Much harder not to worry, or have an opinion.
Time will tell.
For the record, I know a couple of people who've married Thai women... internet 'dating' for a short amount of time, makes me feel uncomfortable. One of the couples though has been together for 23 years, and if I were a gambling person I would have lost a lot of money as they were the couple I thought were least likely to succeed, and 'his' wife is absolutely wonderful... so I cannot say YANBU to think it doomed, because I know that it isn't always the case.
YANBU at all to feel worried, especially for your children.
Personally, though it would stick I would wish him luck and happiness BUT ask him to go slowly with the children, she should be no more than a 'friend' for a while as far as the children are concerned. Perhaps later the complete truth will be the right way, a good lesson that love is weird.