Is "I have two childs" correct?

Why

Ravi has learned the single most useful plural rule in English — add "-s", and it truly does work for the vast majority of nouns: cat becomes cats, book becomes books. The trouble is treating this majority rule as if it applies without exception. A small group of very common English nouns form their plurals in older, irregular ways that don't follow the "-s" pattern at all: child becomes children, man becomes men, woman becomes women, tooth becomes teeth. These irregular plurals have to be learned individually, the same way you'd learn any exception to a pattern, because no spelling rule predicts them from the singular form. Test: try applying "-s" to "man" and say "mans" out loud — it sounds wrong to a fluent ear precisely because English speakers have all internalised "men" as the one correct form through repeated exposure, not through a rule. His sister is right: "children" is the correct plural of "child", and it's one of a small set of irregular plurals in English that simply have to be remembered.