Translation:How many children eat rice?
One obvious, but easy way to remember the translation for "Cuántos" offhand is to associate it with the sound "Quant" or "Quantity" in English, which as you know means how much of something there is.
It works for me, at least.
How many children are eating rice? I got this wrong, but I don't understand. PORQUE?????
Por que, not 'porque'. You got it wrong because the question is asking How many children eat rice?
If they wanted to know How many children 'are' eating rice then it would need to read: Cuantos ninos SON comen arroz?
Actually, ¿Cuántos niños comen arroz? can sometimes mean "How many children are eating rice?" because the present tense can sometimes be used to express the present progressive. HOWEVER, 95% of the time, the present tense is translated directly, like "How many children eat rice?". If you wanted to say how many children are eating rice, you'd say "¿Cuántos niños están comiendo arroz?".