It is common for female Alligators to have multiple age youngsters in her care, which may include some hatched from other nests as well as her own. These two from a pod that included three ages/sizes are a good example.

I stayed focused on the moving Alligator, wondering if there was going to be some action.

There was not; the moving Alligator just plopped down. In addition to seeing the size difference in their heads, you can see the smaller Alligator’s tail just under the larger one’s chin.