The classic worry in Thai households: use two languages with your child and they will get confused, start speaking late, or end up half good at both. Decades of research on bilingual children answer this fairly clearly. Children tell two languages apart from a very young age, and mixing them inside a single sentence, what researchers call code-mixing, is ordinary development that sorts itself out rather than a sign of confusion. You can put this particular fear down.
There Is a Condition Though: Quality Beats Quantity
What the research does warn about is the other side of it. The language a child hears has to be one the speaker is comfortable in. A parent straining through English they never really mastered says less, plays less, tells flatter bedtime stories, and loses the warmth in their own voice. Those are the very things that feed a child's language development. A mother tongue told with delight beats stiff, careful English every time.
The formula that fits most Thai homes: speak Thai as your main language and speak it richly, then give English its own clearly marked slots. Songs at bath time, a picture book before bed, a vocabulary game over dinner. Ten or fifteen minutes at a stretch, with the whole family enjoying it together.
Household English: Ten Sentences Are Enough to Start
You don't need long conversations. Just pick the lines you already say every single day and say them in English instead, consistently: Good morning. Brush your teeth. Time to eat. I love you. Good night. The same sentence in the same situation every day is exactly how your child soaked up Thai, and any parent, at any level of English, can manage these ten.
Leave the rest to media and worksheets. Native speaker voices come from songs and cartoons, reading and writing come from paper, and you sit alongside as the cheering section. That's a division of labour where everybody gets to do the part they are actually good at.