you could just cut around the design and put them onto a new back ground piece if that will work for you- or you could simply add a piece back onto the background you cut- maybe add some more appliques to disguise the add on- but really you probably wont even be able to tell the added piece when it is all done and quilted- it does not have to be perfect- it only needs to be done with love- so, don't bawl---just compromise and carry on- she will love it!
many of our errors just lead to new design features
if you choose to add back onto the area you cut off you could then (after centering/squaring) add a frame border all the way around to distract- but i bet it will be just fine...