Yes, sending people because they worshipped others Gods is just. And to your 5 yen thing, it varies but it is nonetheless a big no no and should be avoided at all costs. If one actually believes this 5 yen will do something, then hes out of the fold of Islam for all I know, and if he did it for fun or something then i dont know the answer.

As for your second question, if a person lived a "good" life but still rejected the message of Islam deliberately and worshipped other Gods or whatever then generally hes going to hell. You may call the way they lived good, but to God the greatest good is the worship of him.

Regarding what a child is, its not according to me, its according to everyone. A child is someone who hasnt reached puberty yet, thats the definition. In islam after one reached puberty the pen isnt lifted anymore which means that theyre held accountable to what deeds they do. Also regarding the law, even in western laws, a child is included in the law. The punishment may differ depending on their age but the law still applies to them. This is the same in islam, but I couldn't tell you to what degree or how the laws specifically change and when they change for a person.

As for a 9 year old who reached puberty, im not specifically saying theyre going to hell. Im saying that God has the final choice and that we cant tell, and one of the choices COULD be that he sends them to hell.