If you don't feel their application would be successful, and the cost of two applications failing does make it a big risk, you could maybe change the circumstances.
Your wife could return to Thailand and take care of them if she feels they are not getting the correct upbringing.
If she has to because their father is away and not supporting financially and their grandparents can no longer cope and she has no other options, then after 6 weeks or so of displaying sole responsibility an application would be stronger.
Obviously she would have to think about her own existing visa (and future applications) and strike a balance of an acceptable time out of the country with a period of proving sole responsibility.
Its certainly not ideal but it's an option.