There are some fantastic core poses in yoga. Try plank and boat (then move from boat to low boat and back). Also put a block between your feet while lying on your back and crunch up to grab it with your hands. Then continue by passing it back and forth from your hands to your feet. You'll feel the burn in no time. Bicycle while in a low crunch is great, too.
You will never have this day again with your children.
Study their little faces and feet and hands. breathe it in.
Enjoy today mama, it will be gone before you know it.