【定语从句】the mother kissed her baby,----was in her arms这里填的为什么是

当先行词是婴儿或小孩时,用which而不用who
例如：The baby which is lying there has been crying for about twenty minutes .
This is the very child which came here five minutes ago
再问： 当先行词是婴儿或小孩时，用which而不用who 这条是固定规则么？