Another possibility is that it's a misnomer or a play on words. Maybe there was a misunderstanding at some point that led to this phrase being used in your family stories. It could be that a small item was thought to look like a part of a rooster in a very humorous and innocent way, and that's how the 'little cock' stories started.
Most mainstream movies avoid such explicit and disturbing content. The movie industry generally adheres to certain moral and ethical standards, and such a plot would be considered inappropriate and offensive for a wide audience. However, in some very low - budget or underground movies that are not subject to the usual regulations, there might be some extremely rare and inappropriate depictions, but these are not the norm.
Such stories often involve very disturbing and unethical content. However, in a civilized and legal society, prisons are supposed to be places of rehabilitation and protection of rights. While there may be individual untrue rumors or fictional accounts, the vast majority of prisons have strict regulations to prevent any form of sexual abuse.
There are many negative impacts. Firstly, it is unhygienic behavior which can lead to the spread of diseases. Secondly, it is not a proper moral behavior to teach, which can distort the child's view of what is right and wrong. Finally, it can also make the child a target of ridicule or misunderstanding from others.
One important tip is to use positive reinforcement. For example, if you're teaching her to suck through a straw, when she does it right, give her a small treat or lots of praise. Another thing is to choose the right equipment. A straw with a soft tip might be more comfortable for her. Also, make sure the drink you use is something she likes so she'll be more motivated to learn.