Australian Migration visa Child’s Police check Subclass 300 and subclass 309 provisional marriage visa and Australian partner visa are a very complicated affair and very time consuming with the average visa application taking anything up to 8 months for a visa decision.
It must be noted that your Thai fiancee or Thai wife will need a Police clearance certificate from the Bangkok special branch which is standard as part of the visa application which falls in line with the Australian sponsors federal Police check.
If your Thai wife or Thai fiancee has children or a child that is aged 15 years or over then they also will need to obtain a Police clearance report from the Thai special branch in Bangkok even if they are not traveling which seems bizarre why you would need to do this when the children do not need a visa but the reason is simple it is just in case in the future if you want a visa for the Thai children then the checks are already in place.
If you require a Police check for your Thai partner you must make sure that you add and supply a copy of your passport signed and stating on the copy that you require a Police clearance check for your Thai partner for an Australian visa application otherwise the Thai Police will not accept the application for the check, your Thai partner must also supply their birth certificate, ID card, Household registration, and marriage certificate and change of name certificates so make sure your Thai partner is well armed with the correct documents.
As far as the children are concerned if the Embassy officer at the Australian Embassy in Bangkok decides they require the child to go for a body medical then they will request it via an e-mail but the Police check you can accomplish this first.