Given that there are precedents of people being detained (for weeks!) for no apparent reason, I assume that the parent is scared that it could happen to them.
A French citizen was denied entry because they found messages criticising Trump on his phone/computer. Not detained for weeks, but could well have been for all we know.
If you refuse then they won’t let you in, and you will be banned from ever returning
https://www.eff.org/issues/travel-screening