佛頂尊勝陀羅尼經教跡義記 2.191

2.191

述曰：第五、佛子住持。言真是佛子者，為能紹隆聖種，故名佛子也。梁朝《攝論》云：「佛子有其五義：一、以大乘為種；二、以般若為母；三、以定為胎；四、以大悲為乳；五、以諸佛為父，所以名為佛子。」真諦三藏云：「成眾生身必須五事：一、父；二、母；三、種子；四、胎；五、乳。若有父無母亦不得，若有父母無種子亦不得，若有父母種子無胎藏亦不得，若有父母種子胎藏無乳母亦不得生，要須具此五緣方始成子。」今者佛子，亦復如是。

Commentary: Fifth, the Buddha's heirs upholding [the teaching]. As for 'a true child of the Buddha': because they can carry on and exalt the noble lineage, they are called a child of the Buddha. The Liang dynasty Mahāyānasaṃgraha-śāstra states: "A child of the Buddha has five meanings: first, having the Mahāyāna as the seed; second, having prajñā as the mother; third, having samādhi as the womb; fourth, having great compassion as the milk; fifth, having all Buddhas as the father. Therefore they are called a child of the Buddha." Tripiṭaka Master Paramārtha said: "To form the body of a sentient being, five things are required: first, a father; second, a mother; third, a seed; fourth, a womb; fifth, milk. If there is a father without a mother, it cannot be; if there are parents without a seed, it also cannot be; if there are parents and a seed without a womb, it also cannot be; if there are parents, seed, and womb without a wet-nurse, birth cannot occur. One must possess these five conditions before a child is formed." A child of the Buddha today is also just like this.