adults can get hand foot mouth disease - HEALTHIES
I am an adults' English teacher suggests that you teach multiple adults. On the other hand, these sentences are both awkward. Possessives tend to work less well when long phrases are involved.
Understanding the Context
In this case, it can become less clear what the possessive is determining: is the subject you teach "adult ('s/s') English" or "English"? Finally, and probably best in your case, you can use youngster (s). Once more, though, this is a term that is used by older people towards younger ones. It is more used for teenagers and young adults than for actual children but it also carries an implication that the person using the term is older than those she is describing.
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Key Insights
As AndrewGrimm notes, "children" has two very distinct meanings: It can refer to people who are not yet adults, or it can refer to people who are the offspring of a specified person or people. It is normal and common to refer to adults as "children" when expressing the relationship. Saying that so-and-so are the "children of" someone is another way of saying the "sons and/or daughters of". For ...