I built a couple of these ( two sets of grandchildren ) and was I surprised how much the kids enjoyed them. They were able to walk and play on the line in a fairly short period of time.

Basically a wood frame covered in padding that is normally used to cover hard living room floors. A simple strap ratchet provides a slack line that will stand the weight of the kids and can be tightened over time.

Step 1:

The wood frame could be made in may ways. I chose pressure treated wood 5/4 thick by four inches wide by eight feet long. I chose this material since it was on sale.

You will need a saw to cut the wood. A hand saw will do.

The first step was to join two lengths together with a strip about one and one half feet long in the middle - giving a three ply affect. Once the main assembly is complete two more spacers should be added between the middle piece and the ends, just to add to the stiffness and strength.

Assembly is with decking screws. In this case the length I used was 1 and 1/2 inches. Using a drill driver and decking screws allowed for easy dis-assembly if something needed to be changed and yet it can be put back together with all the original strength. You can purchase them in bulk at the hardware store. 1/2 pound should be more than enough.

Step 2:

The next step is to add an upright on the end that will stand the strain of the slack line.

I used a three ply method that proved to be strong enough to withstand the downward force of an overweight grandpa.

I attached an upright on each end using a couple of screws on each side of the cross member. I then braced it with an angled piece that was also screwed in place on each side.

The upright was about eighteen inches but dimensions are not critical.
I marked the angles by laying a strip of wood in place and marking with a pencil.
I just used judgement to have the ends be about the same angle but if you cut a second one, using the first as a pattern, it will be a more accurate duplicate for the other end.

Step 3:

On each side of the angled piece I attached another angled strip of wood. This was screwed in place and it gives a total of three braces for the upright.

The first picture shows how the wood was cut. The second shows the assembled cross member with upright in a standing position.

Step 4:

Next I made a triangle shaped support to keep the upright standing.

Two equal length pieces were attached at the top and spread out at the bottom.
The length is best determined by just holding a board in place and looking at the width of the base. I made the first one ( shown here in the second picture ) with a very wide base for stability. It wasn't necessary. It only needed to be about one and a half feet wide to work.

Once the triangle was formed I added cross pieces. The angles were determined by placing a board in place and marking. The screws that went near the end had to be drilled with a small 1/8 inch drill or the wood will split. I made the triangle support pieces in such a way that they could easily be removed from the upright and in this way I could transport the slack line by car.

Step 5:

I attached the padding with coated dry wall screws. One inch.

A helping hand is always appreciated.

I had a tight rope in mind when it started.
I guess I am behind the times. It really was hard on little feet.
I replaced it with a strap which I tightened with a turnbuckle. Much better.

On the second build I used an inexpensive strap wench that I picked up at the bargain store.