"Dw i'n cael y plant heno."
Translation:I have the children tonight.
Technically, that might be "singulative" rather than "singular" since it's derived from the base form (with a plural meaning) by means of a suffix.
There are a number of such nouns in Celtic languages, especially for things which commonly appear in groups, such as berries, ants, trees -- and children.
Perhaps vaguely comparable to things such as "furniture", "information", or "rice", where the basic meaning is collective and the singulative is longer: "piece of furniture, item of information, grain of rice".