Maybe you don't have a go kart. Why not build one? Here are some other instructables on how to build yourself your first kart:
Make your own Go-Kart.
Design and Make Your Own Go-Kart
It doesn't matter how many wheels it has, it doesn't have to be a 3 wheel like mine, it could have 4 wheels or even 2 (mini bike), the same basic principles apply.
Stay tuned because the engine section applies to any small engine, like what is found on lawn mowers, pressure washers, generators, etc.
Step 1: Brakes
Pull the brake lever(s) or push the brake pedal (left pedal) and try moving the kart. You shouldn't be able to move the kart and have the wheels with brakes on them turn. Also check for "slop" in the brakes. The brake should engage when the lever or pedal is just moved a little bit. If you have to pull the lever almost all the way to the handlebar or push the pedal almost all the way down, your brakes need adjustment. Hydraulic brakes are self adjusting. You can tell if you have hydraulic brakes if there is a reservoir for adding brake fluid.
To adjust a brake lever, unscrew the cable retainer from the lever a little bit and try the brake again. The retainer is the green thing in the picture below. You should need pliers when you first start unscrewing the retainer. You might have to unscrew the lock nut in order to get it to turn freely. When it's adjusted right, the brake should fully engage by only pulling the lever a little bit. If the retainer unscrews all the way, this is a sign that you either need new brakes or they need to be adjusted down at the caliper/band/drum. If you don't know what you're doing down there, take it to someone who does. The health of you and/or your children is more important than a couple bucks saved by doing an adjustment yourself.
To adjust a brake pedal on the 4 wheel karts, follow the rod linkage from the petal to where it ends. There should be some threads and a nut there. It should be apparent now how to do the adjustment. If it isn't, take it to someone who knows what they're doing.
You'll also want to check the amount of brake material on the pad/band/shoe(s) that it isn't below the wear indicator or isn't excessively worn. Again, I highly recommend taking it to someone unless you've done brakes before and are confident with your own work.