Despite wool's reputation for being somewhat scratchy, all four of my children have worn it over their (cloth) diapers with great success. My youngest still refuses to give his up and wears has last toddler sized pair of long pants as capris. Wool is stretchy and fits for a looooong time like that.
Although I haven't knit baby wool for a long time, I've got a newly pregnant friend who is considering using cloth diapers, so I decided to encourage her along by knitting some "longies" to go over diapers for autumn baby.
This is the top — you can see the ribbing at the waist, where I put in an eyelet row for a ribbon tie although the ribbing alone will be stretchy enough to hold the pants on. I've just finished putting in a gusset for the crotch and am about to split for the legs and knit them on down.
This is some of my own yarn, called "Sunshine." Sadly, it's one of those playing-in-the-studio-without-taking-notes colors, which I did for fun in this Blue Faced Leicester and in some Seacell/Merino blend Sea Wool as well.
I couldn't resist pulling one skein out to knit these longies with and I think I'm going to have to spend some time trying to figure out just how I did this one.