Man with toddler held up by armed thieves

A man with his three-year-old son has been held up at knifepoint south of Perth.

The man, aged in his 30s, and his young son were walking in Baldavis, 46km south of Perth, when they were approached by two men armed with a knife at 9am on Monday.

They threatened him and demanded his wallet before making off with cash and credit cards, police say.