You're right, but this example shows that Hungarian is a bit more flexible in this respect.
Bare nouns, that is nouns without a definite or an indefinite determiner, are often used in the singular in Hungarian when they are translated into English in the plural:
Virágot vettem. — ‘I bought (some) flowers.’
So when we're not talking about a specific group of children, it's fine to say Nem látok gyereket and Nem látok gyerekeket to mean the same things. When we're talking about a specific group, you'd obviously say a gyerekeket, in the plural!