Trick of the loop is a particular con game. A trick of the loop man would be a two-bit hustler, without the skill and dexterity implied by juggler or legerdemain. The father is implying that the illusionist's purported skills are cheap tricks that only a rube would be fooled by.
Specifically, trick of the loop is a con in which a rope is knotted into a figure-eight. The mark (person who is being hustled) tries to grab the intersection so that when the rope is pulled away, he has it between his fingers. Due to the way it is tied, the mark rarely wins, or only when the stakes are low.
I found a source: http://www.worldwidewords.org/articles/ar-gal4.htm
One gambling game required a leather belt, garter or string tied into an endless loop. The man in charge twisted it into a figure-of-eight formation and asked someone to put a finger into one of the loops thus made. If the string snagged on his finger when the string was pulled away, he won. The trick was that there were two ways to make the figure-of-eight. In one, the game was genuine, with one loop snagging and the other not; in the other, neither did, and the victim always lost. It has been known under many names: pin and girdle, prick the garter, trick of the loop and strap game, but it has been commonly known from the sixteenth century and after as fast and loose, using fast in its sense of something fixed or immovable.
Here's the San Francisco Law outlawing it, from 1903