All of the Koopa kids can be defeated with three jumps to the head or 10 fireballs with Fire Mario.

Larry:

The boss of world one and the easiest Koopa kid to defeat. Larry is the slowest and smallest of the Koopas and fires his wand slower than the others as well.

Roy:

Roy is the boss of world five. Be careful when he pounds the ground as it will stun you and leave you vulnerable to his attacks. To avoid this you have to jump just before he lands. As long as you’re not on the ground when he lands his stun attack will have no effect. Roy will also fire at you with his wand and spin around at you in his shell after you hurt him.

Wendy:

Wendy is the boss of world 3, the loops that she fire will bounce around the room after she fires them. This makes the fight a bit tricky.

Lemmy:

Lemmy, who is the boss of world six, will be riding on a ball when you fight him. These balls can damage Mario, so be careful. He also moves quite fast on the ball so it's a good idea to wait for him to slow down awhile before attacking him.

Morton:

The boss of world 2, Morton is not too difficult to defeat. However the part you may find a bit tricky is when he spins in his shell after you hit him. To avoid getting hit by his spinning shell, jump away from him after you hurt him.

Iggy:

The boss of world 4and fastest of Bowser's children so be careful when fighting him. Stay close to him so that when he comes out of his shell you can hit him.

Ludwig Von:

The boss of world 7, Ludwig is the most difficult to defeat of the Koopalings and arguably harder to defeat then Bowser. He jumps high and fast and when he lands he can stun you. To avoid this jump just before he hits the ground. He also blasts magic attacks from his wand very fast so you will have to be careful to avoid those as well.