Excretion of small kangaroos while in the pouch
Wow! What an interesting question! 

I did a bit of searching around, and here's what I found. It looks like the baby kangaroos, or joeys, do in fact produce excrement while in their mothers pouch. The mother kangaroo uses her tongue to clean her joey and her pouch. 

I found this information in a couple of differently places by doing a google search for "biology of marsupial reproduction pouch excrement."

One of the first sources I found was a New York Times article titled "Marsupail Maintenance." You can find this article at this link:
http://www.nytimes.com/2012/08/14/science/how-do-female-kangaroos-keep-their-pouches-clean.html?_r=0

The article was published in a reputable news paper, and used an interview with a curator of mammals from the Bronx Zoo as a source. 

Another resource I found when researching this topic was an Australian wildlife sanctuary. This wildlife sanctuary has information web pages online to help visitors learn more about the animals that live in the sanctuary. You can find their entry on Eastern Grey Kangaroos at this link:
http://billabongsanctuary.com.au/native-animals/mammals/eastern-grey-kangaroo/

This web pages gives a pretty detailed overview of the Eastern Grey Kangaroo. The information on joeys and pouch cleaning can be found in the section called "Raising the Young."

I hope this fully answered your question!