This trail to Lao Lao Bay starts on Railroad Drive in San Vicente. The entire hike is down a gully that is usually dry, but fills with water when the rains are heavy. Today it was dry. The hike involves a fair bit of climbing over rocks and under fallen trees, so it can be a bit slow. The hike down is lush and shaded. Some of the most massive Banyan trees on Saipan are on this trail.
It is moderately kid friendly. I've taken my kids on it a couple of times. The ones under four usually need to be carried (mostly for speed), but the older ones do great.
To get to the starting point, turn down Railroad Drive, which is near the San Vicente post office on Isa Drive. The trail starts near the second telephone pole. If you get to the first set of houses on the left, you went just a little too far. There really isn't a trail head per se. At the lowest point in the road, just dive down into the overgrown gully and start walking.
You'll pop out, about an hour or so later, on the road to Lao Lao Bay. Walk left on this road, and you'll soon be at the beach. The fastest way to get back is to not go back up the gully, but rather to walk up the Lao Lao Bay road to Isa Drive, then turn right and walk along the main road (Isa Drive) to Railroad Drive.
It's a great hike, and can easily be done in a couple of hours.