I can’t improve on Ven Dhammanando’s analysis below -
“mātā yathā niyam puttaṃ, āyusā ekaputtam anurakkhe,
evampi sabbabhūtesu, mānasam bhāvaye aparimāṇaṃ.”
“As a mother might protect her own son, her only son, unstinting even of her own life, even so should he cultivate a mind [of friendliness], setting no limits with respect to any beings.”
มารดาถนอมบุตรคนเดียวผู้เกิดในตน ด้วยชีวิต ฉันใด พึงเจริญเมตตามีในใจไม่มีประมาณ ในสัตว์ทั้งปวง แม้ฉันนั้น
(Karaṇīyamettā Sutta, Sn. 149)
The English translation will probably look rather different to others you are familiar with, for I have tried to make it clear that “own son” in the simile’s vehicle should correspond to “mind” in the simile’s tenor. That is to say, it is the mettā-yogi’s thought of mettā that is to be acted upon in the manner of a mother guarding her own son. Every other English translation that I’ve seen either misleads the reader into supposing that “own son” corresponds to “all beings” or else leaves the correspondence unclear.
Edit - I’ve put the above in quotes, as I think the readers thought this was my translation, when it was Ven Dhammanando’s.