I assume that you will easily be able to afford to fund the trip so just bank statements to prove this plus a letter of invitation and details about the accommodation. The main problem area is nearly always providing evidence that she will return to Thailand at the end of the trip. This needs to be given a lot of attention with full details and any written evidence in support. For example if she has a job in Thailand then a letter from her employer in English confirming she can have the time off and the job will still be open, This is just an example of one reason to return. Having children in Thailand is not, on its own, considered adequate reason to return.