Oxen and children end in -en because English once had a common -en plural, and these words are the survivors.
First, what is the rule?
Most nouns add -s or -es: cat, cats; box, boxes. A few nouns do something different:
ox → oxen
child → children
foot → feet, mouse → mice, sheep → sheep
The weird part
Nobody says oxes or childs. English learners often try the -s rule on them, and it is a reasonable guess. But the word needs -en.
ox + en = oxen
child + r + en = children
Look at the second one. Children has an extra r that child does not. That is a clue to a strange history.
A very old ending
In Old English, a lot of nouns made their plural with -n. Words like oxa and eye had forms such as oxan and eyen. Over time, speakers also began adding -en to other nouns, so even devil once had devlen as a plural.
By about 1400, -s had spread to nearly every noun. The -en ending faded out. Of the old group, only oxen is a clear survivor.
The double plural
Children is stranger still. In Old English, the plural was cildru, then it became childer. People then added -en on top, and the result was children: a plural with two plural endings.
A word like brethren (an old plural of brother) did the same thing. It is a double plural too.
Men, women and feet
Men, women, feet and teeth look like -en words, but they are different. They change the vowel inside the word: man → men, foot → feet. They do not add an ending. See mice vs houses and women for more.
🗣️ Natural Examples
The farmer had two oxen pulling the cart.
The children played in the park.
There are twenty children in my class.
A team of oxen can pull a heavy load.
The children ate all the strawberries.
⚠️ Common Mistakes
Do not say oxes.
❌ Two oxes pulled the plough.
✅ Two oxen pulled the plough.
Do not say childs.
❌ Three childs came to the party.
✅ Three children came to the party.
Do not add another -s to children.
❌ The childrens went home.
✅ The children went home.
💬 In a Conversation
Sam: One ox, two oxes?
Mia: Two oxen.
Sam: And one child, two childs?
Mia: Two children.
Sam: English really likes keeping old things.
⏳ The OddLingo Timeline
Old English: many nouns have an -n plural (oxan, eyen)
↓ -en spreads to other nouns
↓ -s takes over almost everything by about 1400
↓ ox keeps oxen
↓ child has childer, then gets -en added: children
🥡 The OddLingo Takeaway
Oxen is the last common word with its original old -en plural. Children is a double plural. Everything else follows the -s rule, or changes a vowel as in feet and mice.
See also sheep, deer and fish.