in the Australians In Thailand group. Get the OX visa from Sydney Consulate. It's a 5+5 year retirement visa. Don't pay anything to an agent and no need to transfer money.
My advice is seek out Tod Daniels . But give him your full situation, what your plans involve and he will advise best visa for yourself and your child. Then, and only then, work out the best way to get these visas. You'll find Tod in this group