Split ladder legs for child's playset
best thing to do is install 3/8 threaded rods immediately below each rung.  just drill a 3/8 hole through each stringer such that you can put a threaded rod directly under and touching the bottom of each rung (as close as possible).  run the rod through each stringer and then put a large fender washer and nylock nut on the outside of each stringer.  the compression will hold the structure together and compensate for the cracks in the wood.  should the cracks proceed to spread, the rods will retard the overall splitting of the wood by acting with the rungs as splints for the ladder assembly.  should the wood stringer split vertically from rung notch to rung notch such that the rung could shear through the notch, the rod will act as a secondary buttress.