I am going to break it down into the major components:
1. Yoshi Mascot Head
2. Yoshi's suit and shoes
3. Mario suit and hat
My thought process's (not necessary for the build)
I really like doing themed costumes for my family. Last year we went as members of the Captain N: The Game Master television show. While that was fun and very successful, I wanted to let my oldest pick who he wanted to be this year.
He started off saying that he wanted to dress up as Mega Man again. I talked with him about how much fun we had building the Mega Man costume, trying it on and seeing it become this really cool suit. Then I explained to him we could build something new so we get to have all that fun over again. He was sold.
We did some research trying to work in the themed idea. We got it down to two options: Yoshi or Astroboy. It really wasn't much of a competition, Yoshi was his choice. I was hoping that he would choose Astroboy since it would be pretty similar to the Mega Man costume, but in hindsight I am glad he didn't.
With Yoshi we were going to make his little brother be Baby Mario. Actually my oldest wanted his little brother to be Luigi (I am guessing because the little one has a Luigi plushie while he has a Mario), but the little one really liked the idea of being Mario.
I wanted to dress up as something, but didn't think I would have the time to do Bowser, which seemed the obvious choice. I also didn't want to take away from the kids. So this year I will go without.
With the costumes decided I went to work on design. Baby Mario would be pretty easy, so my main concern was Yoshi. I wanted to build Yoshi to scale, ie Yoshi's head is about the same size as his body. My 4 year old is the perfect size and strength to be able to pull this off. This is why I decided to try and do a Mascot costume instead of a hat or mask. I hadn't seen a costume like this out there, and thought it would be really fun to do.
From there I started working. I failed hard with the first attempted Yoshi head trying to make it out of wire and plastic mesh (it was good, just too heavy for him). But my sons words of wisdom kept me focused.