The six year old, based on your description, is unlikely to have an autism spectrum disorder. Aggressive behavior is not a symptom of autism spectrum disorders and lack of eye contact alone can not indicate an autism spectrum disorder. In any case, a doctor should examine it.
Lack of toilet training would first become an issue when the kid is 5 years old, so as the kid is only 4 she is classified as normal when it comes to toilet training. What regards the language delay, can it be a range of causes for, and should be examined by a doctor.
As you point to, the parenting of the kids should be invastigated as well, since an improper parenting may lead to all the abnormal behavior as well.
Both of the children may well display some developmental concens and these invite evaluation by a Developmental Pediatrician.