"What do girls eat?"
Translation:लड़कियाँ क्या खाती हैं?
When क्या is used as 'what', it is placed right before the verb in the sentence. This means that it would generally come after the subject.
क्या may also be used to turn a statement into a yes/no question. This corresponds to questions using Do/Is/Are etc in English. In this usage, क्या is always placed in the very beginning of the sentence. For example, you can turn the statement तुम अच्छे हो (You are good) into क्या तुम अच्छे हो? (Are you good?).
लड़कियाँ is the feminine plural.
लड़का-Masculine singular, लड़की-feminine singular, लड़के-masculine plural and लड़कियाँ-feminine plural.
You might be confusing them with the oblique case forms which are to be used when the word is followed by a postposition. These are:
लड़के-Oblique masculine singular, लड़की-obl. feminine singular, लड़कों-obl. masculine plural and लड़कियों-obl. feminine plural.