Actually, I had the same problem. Thanks to Roger Carlson at http://www.utteraccess.com/forum/reference-text-box-sub-t1768698.html
I finally fixed it!
For some reason access may have renamed your subform when you moved it onto tabs to “Child##” where ## is some random number. Double-check that’s the problem. I’ll bet it is.
You don’t have to name a tab control when referencing a field on a form. It’s still on that form, whether it’s on the tab control or not.
It’s a relatively old question but I hope it at least helps others who are searching for the answers.