can start early
As one who has been trained in multiple phonics curriculum and have studied early childhood education, you can start kids off in phonics as early as 2 yrs old and yield results. Believe it or not, you might even get better results if you start early.
I won't endorse any particular place or system, but these factors DO MATTER. As you're looking into this, you might want to make sure you the instructor is either American (or Canadian) accent or UK accent. This changes things a bit. American is probably best since it follows phonics rules a bit more.
Also, phonics systems do matter too. The most common in hk is Jolly Phonics followed by Letterland. Both are decent, but not so great compared to more advanced and proven effective systems that have come out from other publishers. And JP is British so just know that it's already biased for the British Accent in multiple cases.