Denise,
So long as you keep the pH at a reasonable level you can keep using your pool during the alkalinity lowering process. In fact, one of the ways Ben recommends aerating the pool is with a group of rowdy kids. At any rate, you should keep the pH one step above the lowest level your test kit can measure to ensure that it doesn't go really low (since 5.0 or 6.0 will look much like 7.0 if 7.0 is the lowest your kit will measure).

You can swim almost immediately after adding acid; it gets diluted very quickly. If you're worried about it, distribute the acid around the pool as you add it or give it an hour with the pump running between adding acid and swimming.