Idle Di, how nice of you to make the project for your friend's baby. Without seeing the batting I'd say trust your gut but keep in mind that baby quilts get washed and dried a lot. I'd probably risk a little thinner rather than a little thicker. It's hard for many of us finding the batting thickness we like both working with and the end results
For ClaudiaM, when we first started safety pin basting before they were in the quilt notion section and used the big diaper sized pins, those could go through anything including a top I made with a thick batt, corduroy, and a satin sheet backing -- but they could also punch holes in fabric with the thickness of the pins, or worse yet snag on a corner of furniture and actually tear a bit .
My modern pins, the ones I like are brass and have the little curve in them, would be too small I think. They are challenging/"just right" for some of my thicker battings as it is.