https://www.ncbi.nlm.nih.gov/pubmed/1923558
Even inhaled gluten is enough to cause immune system reaction: https://www.nejm.org/doi/pdf/10.1056/NEJMc063112
The generally accepted safe threshold of gluten intake for people with celiac disease is somewhere on the order between 6 and 30mg (depending on the study). Even that may be high, there are case studies of individuals where as little as 1mg of gluten prevented recovery of their gut:
https://www.ncbi.nlm.nih.gov/pubmed/15497770
So the parent that asked for separate utensils is not insane; sharing a bread knife between gluten-free and gluten-containing bread can be harmful already.
(A close family member and another close friend of mine have celiac disease, and it is seriously not a joke)
Based on the above calculation, I think it's probably ok to use the same utensil as long as there are no obvious crumbs.
Please let us know what those restaurants are so we can avoid them, and any future restaurants you own.