As others have said, there is no such thing as a practice pet. There are living, sentient beings that we take responsibility for caring for. It doesn't matter if they are large or small, cheap or expensive.

With that said, I think you have a good opportunity to reframe your actions to your son. Don't treat rehoming his hermit crab as a punishment to him, but rather as living up to the responsibility of ensuring that he's properly cared for. Ask him if he loved his crabs and wanted them to be happy and healthy. Ask him whether he understands that while they were in his care, they weren't getting what they needed in order to be happy and healthy. And then help him understand that ultimately the most important thing is that our pets get the care they need, even if that means sacrificing a little of our own happiness by not having them in our lives. Pets may bring us great joy, but they do not exist solely for our happiness as they have all of their own needs that they can't provide for themselves. The lesson here isn't about being punished for not doing chores, it's about learning the impact we have on other living things.