I suppose it would be 'only children' but that does not sound quite right. For example, a schoolteacher might say, "in my class there are seven only children".
I could not find it on my preferred dictionary sites, but I found that according to Wiktionary (the Wikipedia of dictionaries) "only children" is indeed the correct plural of "only child". After a little more searching I found Cambridge's dictionary site confirming it over here.
So yes, "in my class there are seven only children" would be correct. Personally I would avoid using "only children" where possible, because I suspect it is not commonly used and relatively easily misunderstood. In this case I would say something along the lines of "in my class seven are an only child." But if the meaning of the word is clear from the context there should be no objection to using the plural form "only children".
It's just only children.
You can find any number of examples of its extensive use in the situation where that topic is under discussion—just see Roaring Fish's answer. In an article entitled: Here’s Why Only Children Are More Successful almost every use is a plural.
As a suggestion, once the subject has been established, you could use "onlies" or "onlys" in writing - as it's a word created on the spot, you can spell it as you wish.
It's also worth noting that:
Ambiguity is staggeringly common and spectacularly uninteresting in English. "Where did the dog bite you?" hahaha. Who cares?
It's commonplace that multi-word plurals are a bit messy in English. (e.g. "Sisters-in-law") Sometimes you may add hyphens - sometimes not - whatever.
You can state endless examples of multi-word (joke) plurals (hyphenated or non) which can be ambiguous because of the first word(s) or which are infrequently used , so they might "sound strange" if they've never come up for you before.
If you Google using exactly the following phrase:
education studies, "only children"
you'll find billions of examples being used in its plural form.
Occasionally it's hyphenated.
Longer variations like non-only children, non-only-children, and non only-children are common also.
This is a good question!
Doing a bit of research supports Raizin's answer that the plural is only children. Ngram gets results for both only children and only-children, but nothing for only childs or only-childs. I know the weakness of that search ("they are only children!" etc) but a straight Google search also gets:
and many others, so it looks as though only chldren is indeed in standard use and follows the sensible principle of pluralising the element that changes number. We can look at horse women and poets laureate as other examples of this.
I have to say that it sounds very strange to me and I would probably have gone for only childs as the plural, at least until this question was asked, on the basis that it seems less ambiguous. "They are only childs" is clearer that "They are only children".
When in doubt, avoid the problem -- for example, by switching to "In my class there are seven children who have no siblings."
Searching "only-childs" on Google returned "Including results for only-children." As a reader, hyphenating distinguishes "only-children" as a term from "only children" as a phrase.
A standard reference will show you that one definition of only is an adjective meaning
having no brother or sister (Merriam Webster)
It is used just like any other adjective, e.g.:
He's an only child / He's a homely child.
But when used in the plural from, disambiguation may be required in the first case:
They are only children. / They are homely children.
Disambiguation comes from context, which might include explicit explanation. To avoid this, you would have to avoid this correct plural usage altogether.
In some cases, word order can obviate the need for disambiguation. Take your example:
There are seven only children.
There are only seven children.
The first example (yours) states that there are seven children who have no siblings. The second example states that there of no more than seven children (of no particular description).
"Only children" will frequently appear with a different meaning (e.g. "the only children allowed to do that are girls") so I'd argue that "only-childs" is the stronger definition (e.g. "they are only-childs" as opposed to "they are only children", which also has a different common meaning). For reference, Wikipedia likes to use "only children." Gotta love English!
I would avoid "only children". I would prefer "single children".
The term would be sole-offspring.
- The serial killer is targeting sole offsprings in the city. If you are a sole-offspring, you are in danger.
- Due to their one child policy, most of the children in China are sole offsprings.
- Due to their one child policy, most of the children in China are sole offspring.
Note that the plural of offspring can either be offspring or offsprings.
However, if we wish to be a little more word-smart or archaic, we could use
- unigeniture: I have seven cases/children of unigeniture in my class.
- only begotten: I think only-begotten children are more likely to be spoiled.
I would prefer the non-plural form: …, each an only child.