If you or your child are taking BJJ, you need to get the right uniform. ASAP. Why?
Protection
In Jiu-Jitsu people are grabbing that uniform by the collar and sleeves as hard and fast as they can. Grips play a huge role in this sport. A BJJ uniform is made to handle that, and the quilted material is pretty forgiving on the skin. Those cheap standard student uniforms are NOT the same. If someone grabs those super hard and fast they can cut into the skin, leaving what looks like rug burn on the neck, upper arms, and wrists. A proper BJJ gi will eliminate a lot of those types of injuries.
Longevity
A BJJ uniform is thick, quilted, and designed to withstand the rigors of training. Again, people are grabbing at the uniform pretty aggressively. You can buy a cheap $30 uniform, and it will maybe last a few months. They tend to rip along the sides and under the armpits. In Jiu-Jitsu, you are also fighting on the ground. The knees on a BJJ uniform are reinforced, so they tend to last for a long time. My oldest one is over two years old and it's just starting to wear out- and I train all the time. On a cheap standard uniform the knees are usually the first thing to go, I've seen it happen after only a couple hours of training.
The Right Cut
Again, grip fighting is a major part of BJJ. If you have a standard student uniform, chances are that the sleeves and overall cut of the gi will be on the larger side. This is because those uniforms are not specifically made for Jiu-Jitsu. I've gone up against people with giant sleeves, and even my little hands can grab onto those suckers and wrap the material around the wrist- resulting in a death grip that is nearly impossible to break. BJJ uniforms have a slimmer cut to make it harder for opponents to catch the sleeves and collars. If you're a woman, I recommend getting a uniform specifically made for females. The sizing and cut will be different, and you'll be much happier with the fit.
You wouldn't wear hockey gear to play baseball, right? We need to think about uniforms in the martial arts the same way. Plus, if you're looking to compete you'll absolutely need the right uniform. Many major tournaments regulate them and won't let you participate if the uniform isn't right.