Did you pick up and knit on the stitches around the neck? If you did, those can be taken out easily. Then see if the neck hole will fit around her head without the collar. If it doesn't you can take out some of the shoulder seam on the inside and just add the neck to the wider hole. The inside won't look as nice, but nobody sees that, anyway. If it fits the sweater without the collar, then pick up the stitches with a bigger needle and add as many stitches as you need to get it big enough.