I'd probly just make a separate petticoat or half-slip out of something very light w/ the tulle ruffles attached maybe halfway down the skirt...?

I can't see the dress in question, so it's kinda hard to be certain how far up or down.

HTH

Has she tried this dress on the child. Will the child have trouble walking or sitting if it is any puffier. The bride may change her mind if the child has trouble, because a fall in the aisle would be much less worrisome than a less than puffy dress.

Having said that--I'd do like carolb said and make a half slip to be worn under the dress and put the tulle up from the bottom a bit. Actually I would make it longer and put a casing in the top, baste the ruffled tulle so it came about 6 inches from the bottom. Then I'd string an elastic through the casing and tie a knot. Then try it on the child and adjust it to get the proper fit. Then you can make the top casing in the right place and hem

Thank you both for your replies. I ended up adding a 12 inch wide row of gathered netting four layers thick 1 inch above the existing row of tulle. It filled out the skirt nicely and the bride to be is delighted.

Thanks again.