Some nouns occur only in the Plural (like English scissors), e.g. часІ 'clock, watch', дљти 'children'. For such nouns the Genitive Singular rule obviously cannot apply, so you use a collective numeral (двће, трће, чљтверо, etc.), with the noun in the Genitive Plural form. The case forms of collective numerals are shown in Numbers and Numerals: Declension
Тђм бІло двће детљй. There were two children there.
У менї двће часћв. I have two watches.
In the case of the words ребёнок (Sg. only) and дљти (Plur. only), an additional complication needs to be taken into account. Although ребёнок can mean a child or offspring of any age, its primary meaning is 'a young child.' For instance, У неё љсть ребёнок means, unambiguously, 'She has a small child.' The word дљти does not have that connotation, but it does not have a Singular form. Thus, in order to say 'I have one child (or two, three, four children)' without implying his/her/their young age, one has to use the words сІн 'son' or дћчь 'daughter.'
У неё бџдет ребёнок. She is going to have a baby.
У неё двће мђленьких детљй. She has two young children.
У неё одќн сІн. She has one son. (no particular age is implied)
У неё одќн сІн и двљ дћчери. She has one son and two daughters. (no particular age is implied)