CARES does not have a crotch strap. This is because the FAA was concerned that given the particulars of airplane movement the strap itself could cause injury. In their testing the FAA found that if the CARES straps and the lap belt were tightened securely, slipping beneath the belt is not common. With that said, children grow and develop at various rates including the ability to sit upright, unassisted, which is necessary when using CARES. You can determine what’s best for your child.
Sometimes it is helpful to use a thin piece of rubberized mat or sink liner to place underneath their bum. You can also try putting a bolster (or rolled up jacket or towel) under the backs of the legs. This helps bend the knees to help prevent them from slipping down.