According to this article How to mislead with statistics, DHS Secretary Nielsen edition reported on June 18, 2018
“The kids are being used as pawns by the smugglers and the
traffickers,” Nielsen said.
“Again, let’s just pause to think about this statistic: 314 percent
increase in adults showing up with kids that are not a family unit,”
she said. “Those are traffickers, those are smugglers, that is MS-13,
those are criminals, those are abusers.”
A DHS representative provided The Washington Post with the hard
numbers behind Nielsen’s statistic. There were 46 cases of fraud —
“individuals using minors to pose as fake family units” — in fiscal
2017, the period from October 2016 through September 2017. In the
first five months of 2018, there were 191 cases.
That is an increase of 315 percent.
It is not immediately clear what the numbers are specifically related to human trafficking arrests, see U.S. Border Patrol Southwest Border Apprehensions by Sector FY2018 for the raw data. According to statistics for 2016 at Human Trafficking - ICE
In fiscal year 2016, HSI initiated 1,029 investigations with a nexus
to human trafficking and recorded 1,952 arrests, 1,176 indictments,
and 631 convictions; 435 victims were identified and assisted.
and Border arrests exceed 50,000 for third month in a row.
The most recent Bureau of Justice Statistics data as to human trafficking is for 2015 Federal Prosecution of Human-Trafcking Cases, 2015, where the total number was reported to be 1,923 total persons referred for human trafficking prosecution
In fscal year 2015, 1,923 suspects were referred to U.S. attorneys
with human trafcking as the lead charge—39% for peonage, slavery,
forced labor or sex trafcking; 32% for production of child
pornography; and 29% for transportation for illegal sex activity. This
was a 41% increase from the 1,360 suspects referred to U.S. attorneys
It is important to note the conflicting reports that have been issued as to the subject matter of human trafficking, see 2018 NCVRW Resource Guide: Human Trafficking Fact Sheet
In FY 2016, DOJ initiated 241 federal human trafficking prosecutions
and charged 531 defendants. Of these prosecutions, nearly 95% were
predominantly related to sex trafficking. DOJ also secured convictions
against 439 traffickers in FY 2016. Of those convicted, 97% were
primarily involved in sex trafficking.
and the disclaimer of the data held within the report
Human trafficking is difficult to measure. Fear of harm to self or
others, language barriers, lack of personal freedom, and fear of law
enforcement often prevent victims from reporting their victimization
or seeking services. Additionally, multiple agencies are charged with
investigating and prosecuting human trafficking cases, at both the
federal and state level, and providing services to victims. The
record-keeping systems across these agencies may not allow for
cross-referencing, and some victims may appear in the records of
multiple agencies. Because of this, human trafficking reports should
not be used to make generalizations about the scale or scope of human
trafficking in the United States.
where "human trafficking" can be divided into at least several categories including "labor trafficking" and "sex trafficking".
It is not clear if the question attempts to distinguish between sex trafficking, labor trafficking, or human trafficking without any forced attached sex or labor after the "victim" or "illegal immigrant", depending on the perspective is successfully trafficked into the United States; for example, a family paying a trafficker to get them to the U.S. without being forced into sex or labor, or if the premise is that all human trafficking involves forced sex and labor.
What is clear is that the total prosecutions for human trafficking arrests at the border is a substantially low percentage of the total "border arrests".
According to Southwest Border Migration FY2018 a total of 145393 persons have been apprehended between April 2018 and June 2018. Correlating the total number of individuals charged with human trafficking for the entire year of 2018 to the publication date of the Washington Post article How to mislead with statistics, DHS Secretary Nielsen edition, we have 191.
Thus, the math is
(191 / 145393) * 100 = 0.13136808512101683