I agree -- I like my baby quilts at least 36" square and I prefer 36x45 or 45" square.
Adding another border will work. What I would do is baste strips of batting around the outside of the existing batting, just butting the edges together and doing a big loopy whip stitch to hold the strips to the batting -- you want to keep the batting flat when you do this. Add enough batting to get to the size you want plus about 2" extra. If your backing is not wide enough, add a border on the back as well as on the top -- just make sure the seams on the back do not match the seams on the front (it gets too bulky if they match).