A US woman has pleaded guilty to first-degree child abuse over the beating death of her nine-year-old son over a missing piece of birthday cake.

The uncle of a nine-year-old boy who was beaten to death over a missing piece of birthday cake in the US has pleaded guilty to first-degree child abuse.

The mother and uncle of a 9-year-old Maryland boy who authorities say was handcuffed and beaten for sneaking a piece of birthday cake have been charged with murder and child abuse, police said.