You didn't mention the ages of your children. A large abdomen in younger children is often due to the fact that the liver is a full adult-size but the belly is still child-sized. Their bodies will have to "grow up" around the liver.

Young children are also just beginning to develop enough muscle strength to support the body upright against gravity. Throwing the hips (and belly) forward is one way to stay upright until the muscles around the spine fully adapt to this position.

You'll notice that by age nine or 10, most children have "outgrown" the postures you have pointed out. The best way to approach your concern is to encourage physical activity of all kinds. Try not to discourage what may seem like "dangerous" activities.

For example, climbing is a favorite activity of young children and for good reason. The movements required during climbing strengthen arms, legs, trunk, and abdomen. The trunk and abdominal muscles stabilize the spine. Find safe ways to foster all kinds of activities.