"Anak itu bermain dengan kuda kayu."
Terjemahan:The child plays with a wooden horse.
Beacause it's A wooden horse. It's a specific one.
If you say "He plays with wooden horses" (you woud need a "s"), it's any wooden horses, as an habit.
He plays with a wooden horse = tunggal, horse.
He plays with wooden horses = jamak, horses.
But not "He plays with wooden horse" that is grammatically incorrect. For singular, you need an article, either "a" or "the".