I absolutely love the dead bug and all of its variations for working on lower abdominal control with my patients. One of the most important things to look out for in this exercise is to make sure your client is using their lower abdominals and not purely relying on their hip flexors for the movement and pelvic stability.
An easy fix to learn to use the lower abdominals over the hip flexors is to place a ball under the heel. This effectively "unweights" the leg and lowers the demand of the exercise. You can then proceed with all of your abdominal cueing in this regressed position, before advancing to a normal dead bug.
Remember that once the pelvis moves, you've lost lower abdominal control! Keep that ribcage down!