Police said the attack happened at 5pm on Ban Rom Fah Thong-Ban Pha Tung Road in Ban Rom Fah Thong village in Tambon Por of Wiengkaen district.
Thaweesak Yodmaneebanphot, who was driving his pickup with his wife, his son and daughter, is the chief of Tambon Por and president of the Hmong People Club of Thailand.
The attack happened when Thaweesak was driving home from his coffee plantation.
His pickup was riddled with bullets that also cut through the windshield and side windows.
Police said Yia Wongnaphapaisarn, 40, and Chayaporn Yodmaneebanphot, 5, were killed in their seats. Thaweesak and Chaimongkol Yodmaneebanphot, 3, were severely injured and were rushed to the Chiang Rai hospital.