"A woman and a boy"
Translation:Een vrouw en een jongen
'Het meisje' comes from 'de meid', but 'het meisje' is a smaller version of 'de meid'. Every smaller version of words, or so called 'verkleinwoorden', get 'het' as an article. But words such as 'het boek', so with an original article of 'het', don't get their smaller version changed to 'de boekje'. Here too, the article of the smaller version (aka 'verkleinwoord') is 'het'; so 'het boek' turns into 'het boekje'.
Now for context, you mostly use 'het meisje'. 'De meid' doesn't get used often, only between good friends, so I'd recommend using 'het meisje'.