What is the irregular plural form of 'child'?

The irregular plural form of “child” is “children.”

Unlike most English nouns, which form plurals by adding -s or -es, child changes internally and takes the ending -ren. The form developed from Old English and related early Germanic patterns rather than from the modern regular plural rule. That history is why “childs” is not standard English, even though it looks grammatically plausible.

The word passed through an earlier plural, “childer,” before becoming “children.” The extra -ren ending is a remnant of an old plural pattern also seen in words such as “oxen.” In present-day English, only a small number of common nouns retain this type of formation.

A common mix-up is confusing “children” with a possessive form. “Children” means more than one child; “children’s” means belonging to or associated with children, as in “children’s books.” “Child’s,” by contrast, is the singular possessive. The pronunciation also changes: “child” has one syllable, while “children” is usually pronounced with two.