You can see in the map I made, that the creek is shallow right about where the bodies were found (50 feet in from the creek bank.) Google Earth can't tell us exactly what the level of the water was that day... but, you can see the sand bars... so, even if below water level, it is shallow there. I think a person could cross there (not comfortably due to the weather) but, imo, this is the easiest place to cross that creek.
From the place where the bodies were found, imo, the killer would be more likely to head up the hill towards the cemetery and then... there is a path up that that follows the ridge of that hill... where he could head back through the woods towards the bridge and, from there, leave the area by either entrance. He could have also left by way of the cemetery but I think he'd need a ride (either his own vehicle or picked up by someone... taking a bigger chance of being seen though.)
He could have crossed back over the creek and left the park by way of the dirt road that goes under the bridge... imo, I doubt he re-crossed the bridge, itself... too much of a chance to come across other park visitors, imo.
I think the most difficult choice would be to walk along the creek if there is no path there and I don't think there is. In places the banks are quite high with irregular walking surfaces; tangled with brush, fallen trees, branches... Again, I'm not sure if there's an actual path along the creek but, even if there is, it wouldn't be the most direct or "hidden" way to leave the park, imo. I also think it would eventually lead to some climbing, as well.
(The yellow pin marking where the girls were found is approximate. I didn't measure 50 feet in from the bank but the location shown is very close.)
ETA: Walking along the creek bank would also expose the killer to more chance of being observed at some points on a hike like that... (even by people crossing the bridge.)