Player::setHeadBlend

Syntax

player.setHeadBlend(Number shapeFirstID, Number shapeSecondID,
  Number shapeThirdID, Number skinFirstID, Number skinSecondID,
  Number skinThirdID, Number shapeMix, Number skinMix, Number thirdMix,
  boolean isParent)

Shape / Skin First ID: "Mother"

Shape / Skin Second ID: "Father"

Shape / Skin Mix: Float values ranging from 0.0 ("Mother") to 1.0 ("Father").

Third ID: If you feel like another ID should be mixed in. Personally I leave 3rd IDs to 0 and 0.0f mix. Basically they appear to relate to the parent / legacy character customization in GTAO.

IsParent: I'm assuming SET_PED_HEAD_BLEND_DATA can be called again to reference this character as a parent.