It sounds like your abdominal muscles are stretched out from your pregnancies. You should see your doctor to see if you might have a diastasis recti, which is a separation of the abdominal muscles. Those with diastasis recti can usually continue to practice yoga, but it's information that you and your teacher should have to make sure you don't injure yourself.
When coming up to a seated position, first roll over to your side and then push yourself up to sit with your hands. Feel free to avoid child’s pose, especially if it is painful. You can just come back to sit on your heels in a kneeling position instead if that is comfortable. Over time, your abs should get stronger again.
I know it’s tough, but try not to compare yourself to others in your class. We all have physical limitations to work with.
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