"This child is eating my rice."
Translation:Этот ребёнок ест мой рис.
When using "this" to describe some noun, it must agree with the gender of noun it describes. Ребёнок is masculine, so этот should be used. For a neuter noun like яблоко, you would use это. When using "this" to reference a specific object in front of you, as in "This is a child," это can be used regardless of gender. So the example sentence would be "Это ребёнок," and not "Этот ребёнок."