i've cut just about all my ds's onesies into t-shirts.
i lay it on my cutting mat, lay my strait edge over the onesie, and slice off the bottom with my rotary cutter. i don't even serge the edges. i've never had one unravel or roll.

easy peasy!
if you dont have a rotary cutter and mat, just slice off the bottoms with your scissors. i suck with scissors, so my edges would be wonky. LOL!!