When selecting an instrument, the first rule is to always ask your child's band teacher! Many band teachers have strong preferences about what their students play. If the teacher does not have a preference, the following is a general primer.

Plastic or Wood?
Wood clarinets have a better tone, but need more care and cannot be exposed to the elements (such as those your child encounter in marching band). This makes a plastic instrument preferable for beginning band students. Don't worry - the clarinet is one of the most successful plastic instruments out there, and for a beginner it's entirely adequate.

Keys
The best keys should be made from forged nickel silver, as they are not easily bent or broken.

Price
There are many great brands out there and a range of prices for both renting and buying. Do not be tempted by the "bargain" priced brands, as these instruments tend to break quickly. Worse still, most repair shops will not even look at them because they are too cheaply constructed and they don't want to risk it.

Also note that most music stores offer a rent-to-own program. Since you may wish to upgrade your child's instrument or change brands later on, try to negotiate an arrangement in which your rental payments can be applied to any later clarinet purchase in the store.

Brands
Some great brands for beginners include Yamaha, Selmer, and Jupiter. Later on, as your child grows, consider upgrading to a Buffet clarinet (grenadilla wood is the best, but there are some great imitations that are nearly as good).

Clarinet Reed Selection