I bought a pair of TaoTaos last year, A Rhino for me and a 125D for my 7 year old grandson. His little brother is almost 4 and I needed a way for the little one to come along for rides. I bought a MadDog seat off of Amazon on sale based on a recommendation from someone here (I forget who). Coleman sells a seat just like it for about half of what the MDs are going for now. The seat didn't fit well, however, as the rear rack has side and back lips that are in the way. Below are pictures of what I had to do to make it fit. Basiclly, I took off the plastic cover and then cut off all of the tubing that created the lip. I then bent some electrical conduit, making it large enough to support the seat. I then cut and welded support pieces that connected the original frame to the new one. After cleaning it up with a grinder and wire brush, I painted it textured black to match, let it cure out in the sun, and bolted it back together.
A couple notes on working with the conduit. First, safety. The conduit is plated and will give off dangerous gases when welded. The plating can be removed with muriatic (hydrochloric) acid. I always have some in my garage as it's great for removing rust. I placed the support pieces in a plastic container and slowly poured the acid in. I poured the acid back out when the reaction stopped. I brushed acid on the frame where I was going to weld with rags underneath to catch the drips.The second thing of note is that I didn't have a long enough piece of conduit to make the outside of the frame. I spliced two pieces together using 5/8" threaded rod and pop rivets. I found from another project I did that 5/8" rod will fit inside the conduit pretty tight. Once I got the ends together, I drilled some 1/8" holes through the conduit and rod, and then attached it all together using 4 rivets.
After it was all together, I rigged up a seat belt for the young one so he couldn't fall off. It worked but I want to come up with something better this year.