Jack up rear axle, remove wheel.

Have friend apply hard pressure to brakes.

Look all the way around rear drum for cracks. Rotate drum 90 degrees and check again. If cracked, it will open up to where you can see it when brakes applied.

If no crack in drum, pull drum and inspect shoes to see if they are shifting and attempting to rotate with drum.

Check backing plate mounting bolts tight and also for cracks. Look closely at wheel cylinder. See if it is bolted tight and check backing plate near it for cracking.

Make certain shoes are not reversed. Some brakes have leading and trailing shoes. One goes in front, the other to the rear....I don't know if Toy rear brakes are like that or not.

P.S. Also, before removing the rear wheel, spin it to see if it runs true and is seated flat against the drum, that your wheel isn't loose or wallowing at the lug studs due to wrong lug nuts being used.