豆 in 頭 is actually more or less an indicator of how 頭 could be read. This is a common characteristic in many Chinese characters. The onyomi of 豆 is read as とう, as seen in the word 豆腐 (とうふ). Likewise, an onyomi for 頭 would be とう, as seen in the word 頭部 (とうぶ), which means "area of/around the head". Another character that uses 豆 to indicate reading but not meaning is 闘 (とう, to fight) in 戦闘 (せんとう, to fight/battle), but this pattern can be seen not only in 豆 but in other radicals as well.
頁, on the other hand, is an evolved version of what was originally 首 written above 人, which I'm sure makes total sense when it comes to the meaning of the character.
Hope this helps,