Learn Arabic (Modern Standard): كم طفلا رأيت في الحديقة؟ - How many children did you see in the garden?

In Arabic, after the word كم (how many), you must always use a singular noun. Even though the English translation says 'children', Arabic requires the singular form right after كم.

Why does the word طفل have an extra alif added to the end to make it طفلا?

This is another strict rule for كم. The noun directly following كم must be in what is called the accusative case. For most masculine words, this means adding an 'an' sound at the end, which is written by adding an alif.

Where is the word for 'you' in رأيت (did you see)?

The pronoun 'you' is built right into the verb itself! The suffix ـت added to the end of the past tense verb shows who did the action. It is pronounced ra'ayta if you are asking a man, or ra'ayti if you are asking a woman.

Can I change the word order and put the verb right after كم?

No, the noun you are asking about must stay glued to كم. You must build the phrase كم طفلا (how many of a child) first, and then place your verb رأيت (did you see) right after it.