I use backdrops from LemonDrop: http://www.lemondropstop.com/
I get the polypaper, which is the cheapest. It isn't paper like it sounds, but rather a plastic type of material. It is very hard to wrinkle and it also doesn't have any glare. I just tape it to my wall using painters tape. The standard size of 5'x5' is great for a couple of kids, pets or single portraits. You can also get wider sizes as well (5'x6', 5x'7', etc). There are lots of choices too. It seems like kid photographers mainly use them, but there because there are so many choices, you can find appropriate backgrounds for almost any occassion. Check out Facebook or their website for examples. I like these much better than muslin because there are no wrinkles to contend with.
Below is an example of a "Christmassy" background I used to photograpy my boys.
|Antz by Deadfisheye|
from Animated Film Title
|The Ladder - Fort Worth Museum of Modern Art by RJD13|
from Lookin' Up!