As all of us know as a general grammatical rule,the verb tense in second clause which comes after a simple past tense clause must be in one of the past tenses.

For example:  

> She knew she had made the right choice.  

> I touched the baby's head who was crying.  

But I wondered when I saw this sentence.  

> I heard a little boy waving his hands above the water. 

> I saw a little boy drop his ice cream 

Shouldn't it be:  

> I heard a little boy was waving his hands above the water? 
 
> I saw a little boy had dropped his ice cream.   

Are there any exceptions to this grammar rule? If yes,what are those specific verbs ?