Sounds like you're on your way! As I commented yesterday, I modified black sweatpants with a bib and straps (and, yes, big yellow buttons!) for Waluigi. Waluigi wears brown shoes--and my son approved of wearing his black school shoes. I don't know what I would have done if he wore green shoes like Wario! Maybe spray paint some old sneaks? The hats can be bought, but, again, converting a baseball cap satisfied my (then) 9-year-old.

Green slippers: get the next size of Crocs? He can wear them next summer :-).... Or is that going to clash with his yellow- preference too much?

Green shoes? Just make him some spats that he can put over his regular shoes. I wore a lifetime's worth of these with band uniforms and they are not too annoying or dangerous to walk in. I'd go for felt so you can get the upturned toes, too. Now, I wanna know if you'll be knitting big honking mustaches for 'em?

If you make the waistband super big, you can buy a length of pvc irrigation tubing at the hardware store and slip it into the waistband fabric tube. Heat the ends of the pvc w/ a hair dryer to soften, then stick a pvc connector in to complete the circle (pics here: http://jasonunbound.com/hoops.html), hand stitch the waistband shut, and you'll have a nice round Wario belly. We did this for Tweedle Dee and Tweedle Dum costumes once. Worked great.

I second the spats idea - shouldn't be too hard to make - just 2 identical halves with upturned toes. Polar fleece would have a little give & so be more forgiving but felt might be easier to wear (there is the risk that polar fleece spats could flap around - but, if you make them so they fit over shoes a bit snugly that shouldn't be a problem.)

Halloween is the greatest when one's kids are little. I miss it. (I think I say the same thing to you every year!)

Kwik Sew 2426/2427 depending on size. I found it at http://sewing.patternreview.com/cgi-bin/patterns/sewingpatterns.pl?patternid=2310

Overalls aren't in style anymore? Well shoot, I remember making some when I was in middle school. I'm guessing that a search for slightly-vintage patterns (late 80s, early 90s) would find you a pattern for some kid-sized overalls.

Of course, figuring it out as you go along is always fun, too.

Not exactly the same, but I've looked for a similar shoe pattern some years ago. In case you need one, here is what I found.

http://sewing.about.com/od/hallowee1/ss/elfshoes.htm

Happy sewing!

Coolness, May! Maybe I can use that pattern to make spats. B's costume last year had some felt shoe covers to look like superhero boots. I can use the same idea but make 'em look like the funny elf shoes!

If you look on Knitty for the Fuzzyfeet pattern, knit and felt them then spray heavily with Plastidip (one of the brands of Dip-it type products get it at hardware store) He will have his green shoes for his costume and warm felt slippers for the winter.

There is a pointy toe bootie in Weekend Knitting by melanie falick....

For the 'big' pants, I would use a slightly stiff plastic hose that is thin... slip it into the waist band, duct tape ends together... and then sew the t-shirt to the waist band. Slip a zipper up the back of the t-shirt and you have a no-slip off costume. My brothers were tweedle dee and tweedle dum as kids one year and my mom came up with this idea b/c their overalls kept falling down and they were tripping. And, this way, they didn't need to be stuffed.

I ran across these cute Mario, Luigi and Toad costumes this morning.
http://crapivemade.blogspot.com/2008/10/happy-halloween.html
And here's how she did it.
http://crapivemade.blogspot.com/2008/11/details.html

The green shoes... looks like you next knitting pattern!

My husband was Mario one year (I got to be Peach). He wore overalls, a red t-shirt and bit white gloves. I actually found an old Mario pattern in order to make the hat. We used black felt for his mustache and eyebrows and just sort of taped them on.

I'll look around my patterns to see if I can find the hat, maybe it'll scan well and I can share it.

I don't think I would ever feel up to making the overalls, but after spending one of last year's G&S shows as a pirate (complete with moustache) I feel like I could make a decent stab at moustaches.

Strange what things you can learn.

That's so cool! Your kids come up with great costume ideas every year, and you come up with great costumes! It's so much fun to see what you-all come up with.

You could also use the knitty Fuzzy Feet pattern to make felted slippers they could wear in the house after! Are they going to wear great big fake mustaches? That'll be awesome!

Last year for my son's Pikachu)Pokemon) costume, I bought him a pair of adult socks in the color that he needed, and put them over his regular shoes. They were ruined by days end, but I haven't had much luck with spats and other shoe patterns.

Love your kids' costume ideas! I need to get my boys to make some decisions, especially since I usually make their costumes, too.

I knitted a mario, luigi and wario hats for my boys. It's basically the Topi hat from knitty.

Here's the link to my ravelry page with pictures of the hats.

http://www.ravelry.com/projects/xaessedai/topi-2

all good things