I would argue (as would most skiers) that the boots are the most critical piece of gear for a skier. The boot acts as the interface between the skier's body and the rest of their equipment; without a proper interface, it is impossible to control your bindings and skis, regardless of how nice they are. Boots are also the number one reason skiers are uncomfortable: poor fit, hot spots, and cold feet can cause misery for your athlete, and can significantly affect how your athlete perceives the sport.
Therefore, if I'm going to spend money on a piece of equipment, it will be on boots. It is really important that they go to a good bootfitter to assure a proper fit; this isn't like getting a pair of sneakers! There are a lot of variables that go into properly fitting a boot. A professional is your best bet.
This is also the reason why I am often reluctant to buy used boots for my kids. One, there usually aren't any good boot fitters at swaps or on ksl.com. Two, you don't know the condition of the boots. The linings can be packed out or the shells could have been modified, which can affect the performance of the boots. The soles could also be worn to the point that they aren't safe (won't interface with the binding properly, leading to early release problems). Ski race boots, even on kids, get worked hard, much harder than recreational boots!
I have a few other points in regards to boots. First, the most important feature of a boot is fit. Different brands fit differently. This isn't as important an issue in kids as it is in adults, but if your child has wider, high volume feet, consider trying different boot brands. Some are known to fit wider feet. Again, this is where a skilled bootfitter can really help.
Second, it is tempting to get boots that is a size or two larger so that your child can get a few seasons out of it. Considering how expensive race boots are, I'm tempted to do the same thing! However, a boot that is too large can potentially affect your racer's technique and progress. A boot that is too large will not be responsive. This can really make a difference as your racer gets older and stronger. So even though it may be hard financially to get new boots every season, a well fitting boot really can make a difference in your athlete's development as a racer. Also, if you need to get your athlete boots at the end of the season, it's tempting to take advantage of end-of-season bargains. But consider how much your athlete will grow over the summer. The boots you purchased in April may no longer fit in November! It's best to wait until October or November to purchase boots to assure that they fit.
Third, many parents don't consider the flex of a boot when they purchase them for their racers. If your racer is big and strong, get stiffer boots. However, many smaller and lighter racers get stiff boots because they are the "race" boots that look like the ones their skiing heroes ski. Make sure you get your athletes boots they can flex, even if they're not the top-of-the-line race model! If not, they may not be able to drive their knees and hips forward in a good racing stance, and can cause them to sit back. Also, be aware that even though your athlete can flex their boots in the warmth of the store, the boots can stiffen quite a bit when taken out in the snow.
Finally, if your athletes is in the developmental stages (ages 9, 10, 11 and on), consider getting a footbed for their boots. Ever see the stock footbeds? They are no better than cardboard. You don't have to get expensive custom footbed, since kids' feet are constantly changing. However, aftermarket footbeds can really help your athlete's boot fit better and offer more support, which will help immensely in their technique. They aren't terribly expensive (usually $50-$100), but can help immensely.
