The Box Set
The majority of clubs for kids come in preboxed sets that include a couple of woods, some irons, a putter and a bag. Boxed sets are age or height specific. Almost every major manufacturer offers a boxed set. Nike, TaylorMade, Cleveland and Ping all have boxed sets. U.S. Kids Golf is a great brand that specializes in kids' clubs, offering a wide range of boxed sets. Other brands, such as RAM and Lynx, also have children's boxed sets.
For individual clubs, U.S. Kids Golf has the widest selection and is noted as the number one brand in junior golf worldwide. You can buy virtually every club by itself in every size that they offer. Depending on the store you go to, the selection might not be so vast. However, you can buy everything online, along with bags, shoes and clothing just for kids.
U.S. Kids Golf is the only brand out there that sells single clubs for kids. Big-name brands don't have the market base to sell off-the-rack single junior clubs. You may be able to special-order a single club as a replacement, but the odds are unlikely with boxed set purchases.
Although custom fitted is highly recommended for adults, because kids are growing at such a rapid pace, custom fitting isn't the most cost-effective option. But if you're keen on getting your junior player custom-fitted clubs, that is also an option. You would have to see a club fitter and place a special order; you would then get your clubs in a few weeks.