Lacerte diagnostic says: "The Relationship selected is invalid for electronic filing purposes. The following are valid relationships: Son, Daughter, Foster Child, Grandchild, Grandparent, Parent, Brother, Sister, Aunt, Uncle, Nephew, Niece, Stepchild, Half Brother, Half Sister, Stepbrother, Stepsister, None or Other."

So it's that the IRS only recognizes nonbinary people for some categories of relative.

Thanks, I appreciate it!  That's a bummer and will probably result in a few of my clients paper filing instead of e-filing.

Why does it have to be child?  Can't you use one of the other choices?

I work with a lot of trans and non-binary clients that don't feel comfortable using gendered words like "son" or "daughter" for their small children.  It seems weird that the IRS would require one or the other when "child" is a perfectly adequate descriptor of their relationship for tax purposes.

I plan to let them know that their options are, unfortunately, to either pick one or file on paper.  

That's all you can do then is explain it to them and let them decide how to proceed.  Hopefully this will get changed in the future so it won't be a problem.

Why does it seem weird that IRS would require that?  Are they otherwise at the forefront of 20th-Century thought?

They certainly aren't!  It's just that they call it the "child tax credit" not the "son or daughter tax credit".

The Internal Revenue Code uses the word "child" everywhere I look.  I can't find "son" or "daughter" when it defines qualifying individuals for credits or deductions.  Of course, that's just how Congress and the President decided to write it.  If some nameless IRS bureaucrat decides gender is important, then their opinion prevails.  Don't ask for a reason.  If they could reason, they wouldn't be working for the government.  

I seem to recall that at one time child was acceptable.