I have always done a quick transition period WITHOUT mom present.
The week before they start, I have them come on the Monday for a couple of hours.
Then on the Wednesday if possible for a half day including lunch.
On the Friday before they start I ask the Mom to bring them in from about 11 (after we get home from our play time) and pick them up around 3:30.
It doesn't necessarily stop them from crying when they come, but it gives me a chance to know their moods a little better and for them to recognize the other children.