Your kid can smash all those commercial tops with her own homemade one. You can also teach yourself and others about the factors involved in rotational inertia and angular momentum.

You'll need:

Wood
5/16" dowel
Fender washer to match the diameter of your hole saw
Long staples for heavy duty stapler, around 3/4"
Brads slightly shorter than your wood is thick

Hole saw
Saw
Drill with bits: 5/16, 19/64, 3/16
File

Step 1:

Cut a circle out of wood stock with a hole saw. We used the the one with about 2 inches radius.

Sand it smooth.

Mark around the hole with a penny, and a diameter line for placement of the staples later.

Step 2:

Drill out the center hole to 19/64". This will make the fit nice and tight. If you drill to 5/16" you may have to glue the dowel and it may eventually come loose.

Decide on the height of your bayblade and cut the 5/16 dowel to that length.

Pound it into the wood circle from the bottom, so that it is flush with the top.

Step 3:

Drill out the fender washer hole to 5/16" - no need for a tight fit here.

Drill two small holes for brads. (Those two steps not shown.)

Slip the washer over the dowel (on the bottom of the wood circle).

Pound in two brads to hold it in place. It can also be hot glued into place, but will eventually fall off in the heat of action.

Put two or three fender washers if you think it will make your bayblade more lethal.

Step 4:

Put the staples in right on the line, one each equidistant on each side of the center hole, now filled with the dowel. The circle of the penny is where the (commercial) launcher hooks will be, so each staple needs to straddle that circle.

If the staples go in too deep, pry them back out with a flat blade screwdriver. Short staples won't cut it - they'll need to be sticking up about a 1/4 inch, and sunk in as deep as possible.

File your performance tip to the shape of your choice, ruthless, efficient, deadly.

Step 5:

Dangle the bayblade from the launcher hooks and let it rip.

Since it's more massive, it will have more momentum, but also may cause the launcher to skip some teeth and eventually screw up as it accelerates it into action. Go easy. We're still working on a homemade launcher.

File the tip to a different shape for different action. Flat tips tend to travel, curved or pointy tips tend to stay put.

File grooves or build up texture with hot glue if you think these will give advantage.

Tall bayblades' shaft will contact the commercial ones' edge - a distinct advantage. If you try to stop a spinning object (your miserable opponents' b-blade) by touching it at a large radius, you'll be able to exert more force (torque) to slow it down. It, on the other hand, will be touching your (highly superior) b-blade at only the radius of the shaft, thus having far less torque to slow yours down. Resistance is futile.

A long spin is determined by many factors: mass, radius, air resistance (smoothness), balance, friction with the ground, collisions, and more. In the area of balance, the commercial ones have a slight advantage - the machine that spit them out likely made them perfectly balanced. You will need to optimize all the other factors to achieve victory, and that you will surly do.