How tight should you tighten your spokes on a 20" summit trials? We went to a local bike shop, and he said it is really hard to tell, and that is really tricky… any advice?? 
Go to another bike shop.
That is the best advice. Not all bike shops are experts at wheel building. Find a local bike shop that is known for their wheel building skills. Find the shop that the local tandem bike riders go to for their wheels or the shop that the local mountain bike riders go to for their wheels. Tandem bikes and the serious mountain bike riders put a lot of stress on their wheels and they need their wheels to be built well. A shop that does high quality BMX wheels for dirt jumpers and street jumpers should also be good.
I found a local shop that caters to the freeride and DH mountain biking segment and the shop owner does an excellent job of building wheels. He’s awesome. The wheels he builds sing when you pluck the spokes. I’ve never had a problem with one of the wheels he’s built. His wheels stay true and strong and the spokes stay tight. I’ve broken a couple of spokes, but that has been due to me crashing and landing right on top of the spokes and not the fault of the wheel build.
The spokes should be tight enough so that when you pluck them they make a nice tone. If they just go “thud” when you pluck them they are too loose.
One of the wheel builders here in the forum could give you a number for the recommended tension. I just go by tone when plucking the spokes.
When you find a shop that kown how to build a strong wheel let them know that you are going to be rough on the wheel, jumping off of thengs, etc. That way they’ll know that this unicycle is going to get more abuse than your average unicycle and they’ll know the wheel needs to be built better.
AFAIK there is no available tensiometer that will measure the short spokes on a 20" wheel. What I do is get the same tight feel as I have on a larger wheel (which I can measure), when I squeeze pairs of spokes. That takes care of absolute tension. Relative tension is done by tone. The quality of the rim makes a difference as to how high the tension can be.