My Mum had 4 kids and had a great, thin, toned belly when I was a kid - but she was in a running club (running 4 times a week) and also swam three times a week.

She didn't have the crinkly belly because she was so toned. But she was obviously training like an athlete at that time and it took awhile to get there. She wasn't so toned or as active until I was at least 10y/o.

As for the linea nigra, apparently that fades in time.