This post was inspired by my sister-in-law. Several family members, neighbors, and friends have recently had babies. Probably the hardest thing about working out post baby is finding the time and energy to do it. Lack of sleep is a killer.
A 20 minute workout can be a great workout. Even a 10 minute walk everyday helps build your strength.
For reasons obvious to any mother, Do Your Kegels. This exercise will help you as you add intensity to your workouts. I have had several mothers say they can't workout hard or jump because they will have an accident. It seems silly, but it might be a solution to that problem.
Push-ups on your knees. Wide arms or triceps push-ups.
Bodyweight squats can be killer. Hands on hips while you squat. Hands in the air. Touch the floor then reach for the sky. Lots of options with these.
Ab twists--hold a squat position and punch straight forward (jab) or across the body--twist at the waist (cross). Squeeze abs while you do this move. Try other standing ab exercises.
Start easy, be patient with yourself, be consistent.