Echoing what some others here have said, with some comments:
Hermit crabs are great! But be prepared - sometimes they live forever, and sometimes they die quickly. And when they die, you have to deal with that with the child. Not necessarily a bad lesson, but you should be prepared. Also, most people just stick them in a fishbowl or small aquarium and call it good. But if you want any success at all with keeping them alive, there are a few things you need:
- several shells of different sizes (same size and larger than the one they are currently in) so they can change as needed when they molt and grow.
- Water. I know that sounds like a no-brainer, but... they actually need a somewhat moist environment, even though they are land creatures. It's best to keep a (somewhat) large but shallow dish of water in with them, make sure it is ALWAYS full, and keep a spray bottle of water handy, and spray their "home" with it frequently. To help with the humidity, keeping a small sponge in their water dish will help. Also, its better to use filtered or distilled water.
- Heat. Especially in winter, most homes are actually too cold for hermit crabs. Look for small heaters at the pet store that are used for reptiles. Putting a small one of those under whatever you use for the crabs is a really good idea. They require around 75 degrees (F). A heat lamp or keeping them in a window is not a good idea - they CAN overheat, and the window would not provide a consistent enough heat source anyway.
- Something they can dig down into and room to move around. They need to be able to dig and climb.
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Bettas are a good "starter" fish, and interesting too. But if you do go with this, instead of putting it just in a bowl, you'd be better off with one of the "mini" aquariums. Even though they are anabantoids (take in air through gills and can breathe by taking in air through the mouth), they do much better if the water has a filter to provide oxygenation. Without that, you should be changing the water every day or two. Also, the caveat about dying comes in here as well... with hermit crabs and fish, it's easy for something to go wrong and you not have any idea until the creature has died. But these are great fish - almost as hardy (read, hard to kill) as goldfish, but very colorful, and can be very responsive - they learn quickly to beg for food, believe it or not.
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And cats - they are actually probably about one of the easiest pets to take care of. I agree strongly with not getting a feral or even one that had a feral as a mother - they can be great and very affectionate, but getting them to that point can take a lot of work, and even then, they can be a lot more temperamental than other cats.
And I can't agree enough with the recommendation of Maine Coon - add to that Norwegian Forest cat. Of course, getting one from a breeder would be expensive, but they are some of the most friendly and playful of cats. But shelter cats can be great - and sometimes you can find specific breeds there. The most important thing, though, is to meet the cat in person, preferably with all family members, and see if the cat "meshes", personality-wise, with the family.
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Finally, before you make any decision, do a quick search on whatever animal you are thinking about to get a better idea of what the basic care and requirements of that animal are.
[This message edited by osxgirl at 3:10 PM, November 21st (Thursday)]