Not an easy question Cozza. I've raised two half Thai kids in the USA and am raising one here now who is 4.5. The first two was a different era and mentality and being young and foolish, my wife and I put little emphasis on teaching them Thai. Subsequently, as adults they now know little native Thai, only Thai from courses they've taken as adults.
Thinking back, which may be the same as now, unless their Mother took full control to speak Thai to them everyday and subsequently taught them to read/write Thai when the time came they would have had no other support. There was no Thai community, no Thai friends, no bilingual nursery or schools. It was full absorption into American English speaking society. As we've mentioned on here before, you only have about their first 7 years until their on-board automatic language learning computer turns off. After that a language has to be taught as a second language.
Turning the switch to over here now, it's not cheap. A good international school you'll be paying a minimum of about $6,000 USD per year all in for the early years, nursery, kindergarten years. It goes up from there as they get to Primary and Secondary. Once again the Mother is still left with the primary job of teaching Thai in the home, supplemented a bit by available Thai courses in school and after school, and their exposure to the Thai community. It's easier here but not a slam dunk. You still have to work at the Thai side as all of their instruction daily is in English, they tend to watch English TV and converse in English if they have classmates as neighbors.
Our little one is only getting about an hour per week in formal Thai instruction in school and this will stay about the same for the next four years of classes, through grade 4. Next year we're going to have to put her in an after school program daily so she catches up with where a Thai 5 year old should be concerning reading and writing. Speaking is coming along naturally on it's own.
Now, there's also the option of Thai private schools which are less expensive than International schools, I would say about half the cost. The shoe is on the other foot now though (yours) as they do get English instruction but only a few hours a week. All their activities, TV, communication tends to be mainly in Thai. You'll have to force English in your Thai home and probably put her into supplemental English courses when she reaches the first grade level.
It's really not easy regardless of which way you do it and where you do it.
A really important thing, perhaps more so than learning two languages, is what do you want for her future? Will she do her university studies and make a life for herself in the west or here in Thailand? If you and your wife are in a position to make that choice and the answer is in the West, then I would educate her in Australia and let her learn Thai there the best she can. The opposite, if you and your wife foresee that she'll have a good future and adult life here in Thailand.
Most certainly you can do it the other way around and both ways. It just seems better to me that if you can make a life choice early, it's better for the child in the long term. Pete
