Both of the children may well display some developmental concens and these invite evaluation by a Developmental Pediatrician.
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.