Without knowing what shrub they are, it's a difficult one to answer properly. But I would suggest you leave them until they are a couple of foot high. You could trim either side when they get dense enough to make them 1' wide. Could you describe the leaves or may be take a pic the small ones or one of the parent and post on here. There are so many different hedges that are common. It could be privet, box, lonicera etc. Once we have something to go on, we can help you with the time it might take to grow and how and when to prune it.