Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Children
COMP3DTAG_CHILDRENParameter: Cache
COMP3DTAG_CACHEParameter: Anchor Point
COMP3DTAG_MIDPOINTParameter: Solid
COMP3DTAG_SOLIDParameter: Size X
COMP3DTAG_SIZEXParameter: Size Y
COMP3DTAG_SIZEYParameter: Color
COMP3DTAG_COLOR