Do I need a heavy jacket for the north?

If you are visiting Chiang Mai or the mountains in December/January, it can get chilly at night (around 15°C/60°F). A light fleece or denim jacket is sufficient.

Can I just buy clothes there?

Yes. If you land and realize you have too much, donate the extras and buy local elephant pants or linen shirts at any night market for a few dollars.

Packing for a Family Trip to Thailand: The Essentials

Focus on lightweight, moisture-wicking fabrics and pack strictly for a 7-day cycle, as laundry services are cheap and available everywhere. You only need one set of 'temple-appropriate' clothing per person to cover all cultural site requirements.

Prioritize natural, breathable fabrics. Avoid heavy denim or thick synthetics. Stick to linen, thin cotton, or technical athletic wear that dries quickly in high humidity.
Designate one 'Temple Outfit'. Temples require shoulders and knees to be covered. Pack one pair of loose trousers or a long skirt and a lightweight shirt that covers the shoulders for each family member.
Minimize footwear. Pack one pair of comfortable walking sandals (with ankle straps) and one pair of lightweight sneakers. Don't bring heavy hiking boots unless you have a specific multi-day jungle trek planned.
Use laundry as a strategy. Pack for 7 days, even if you are staying for 14. Almost every street in Thailand has a 'wash and fold' service that charges by the kilogram, usually $2–$3 USD.

Do I need a heavy jacket for the north?: If you are visiting Chiang Mai or the mountains in December/January, it can get chilly at night (around 15°C/60°F). A light fleece or denim jacket is sufficient.
Can I just buy clothes there?: Yes. If you land and realize you have too much, donate the extras and buy local elephant pants or linen shirts at any night market for a few dollars.