The most humane method to euthanize is rapid destruction of the brain. This doesn't have to be gruesome. A toothpick can be used with a small fish, for large fish a paring knife will do. Because of the rapid death, very little blood is released. There is a slight spasm, but that is to be expected with any euthanasia that doesn't cause paralysis.
I do understand the child angle, but compassion toward another's pain, suffering, and fear will sometimes require a person to step out of their comfort zone. The right decision is not always the easiest to do.
The otherside of it, rapid destruction of the brain is likely the only legal method a hobbyist will have availble. Often the only other legal methods are with highly controlled and restricted drugs, such as MS-222. Your local laws may vary.
Interesting. I did not know about this. Thanks.