Are there two different ways of using “are” in the question from of the 3rd person plural: “yw” and “yn”? In one sentence the welsh is “ifê eich plant chi yw rheina?” and in another “yn nhw?”.
Yes - in your first example, it’s yw because it’s an emphasised question (are these your children) and because ‘rheina’ is not a pronoun so is treated grammatically as 3rd person singular.
In the second example it’s ŷn (from ydyn - “ydyn nhw?” would also be correct) because it’s a normal question with the 3rd person plural pronoun.
Thanks very much Siaron.