der Bub
boy
A term for a boy used mainly in southern Germany, Austria, and Switzerland.
Der kleine Bub spielt im Garten mit seinem Ball.
The little boy is playing with his ball in the garden.

boy / jack
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boy
A term for a boy used mainly in southern Germany, Austria, and Switzerland.
Der kleine Bub spielt im Garten mit seinem Ball.
The little boy is playing with his ball in the garden.

The word 'Bub' (boy) is mainly used in southern Germany, Austria, and Switzerland. In northern and central Germany, you hear it less often.
In Bayern nennt man einen kleinen Jungen oft liebevoll „Bub“.
In Bavaria, a little boy is often affectionately called 'Bub' (boy).
You can always tell from the context whether a boy or a playing card is meant. If it is about family and children, it means a boy. If it is about card games, it means the playing card.
Mein Nachbar hat einen ganz aufgeweckten Bub.
My neighbor has a very bright boy.
In card games, 'Bub' (jack) is often combined with the suit of the card. For example, people say 'Herz-Bub' (jack of hearts), 'Karo-Bub' (jack of diamonds), or 'Kreuz-Bub' (jack of clubs).
Mir fehlt nur noch der Pik-Bub für eine vollständige Reihe.
I'm only missing the jack of spades for a complete row.
girl
'das Mädchen' and 'der Bub' both refer to children or young people, but of different genders. 'Mädchen' means a female child or adolescent. 'Bub' means a boy and is used mainly in southern Germany, Austria, and Switzerland. In many parts of Germany, people are more likely to say 'Junge'. So the contrast is female for 'Mädchen' and male for 'Bub'.

Auf dem Spielplatz hat der Bub seinen Ball verloren.
At the playground, the boy has lost his ball.
Im Wartezimmer liest das Mädchen ein Bilderbuch.
In the waiting room, the girl is reading a picture book.