It's possible to use both child and kid to include adults (with kids being slightly less formal). Indeed, Mari-Lou A's comment links to the Oxford Dictionary entry for "child" which has a definition:
A son or daughter of any age.
‘when children leave home, parents can feel somewhat redundant’
As a parent, you might have mixed feelings about encouraging your child to move out.
Here are five strategies to nurture the friendship during your kids' 20s and beyond: ...
See also the Wikipedia page for child and the question Referring to adult-age sons and daughters as children.
However, there is ambiguity of "child" also meaning "minor", which we avoid by saying adult child or some variant, such as:
Over a third of these parents said their children cannot afford to live on their own, and more than 77 percent said they don't want their grown children to live with them.
However, adult child is slightly suggestive of immaturity (along the lines of "an adult that acts like a child").
Most women/men find this highly unattractive and hence the adult child generally stays single either forever or until they finally do something about their situation and change their life.
It's also possible to circumvent expressing gender by writing e.g.:
My 22-year-old has returned to live at home, and I couldn’t be happier.
Regarding offspring with no gender, we might append agender, such as in:
Full of compassion, love, and many many questions, Libby shares 5 realizations she learned on her journey to understanding her agender child.
where indeed the agender person wrote:
– your oldest kid