"How many kids are there?"
Translation:Il y a combien de gamines ?
"Il y a combien de gamines?" c'est du français parlé. La bonne formulation est "combien y a-t-il de gamin(e)s?". Refuser "enfant" malgré la légère distinction de sens (enfant/gamin ; child/kid) me semble exagéré compte tenu que les dictionnaires donnent souvent les deux traductions.
Nevertheless, Duo has opted to point out the difference between "children" and "kids" in that the latter is an informal word and not always appropriate on every occasion. Some people would never consider using the informal word while others always use the informal word and cannot understand why anyone would ever say anything else. In this learning environment, Duo has chosen to teach this difference.
It's stays "combien de"+ noun, not "combien des" + noun, whether the noun is plural or not. It works the same with "beaucoup de", "moins de" , "plus de", etc.
They are fixed expressions and do not change with the noun. But be warned, knowing French, there are probably some idiomatic expressions that break the rule for some reason or another.
Interesting that you took that interpretation. There are two different meanings. The first uses the expression "il y a" in the sense of "there is/are" not in the sense of a location, but in the sense of literally "how many" (exist). I.e.,
- Il y a combien de gamins ? or Combien de gamins y-a-t-il ? This question is asking for a "total count" of the kids, not how many of the total number are at your location.
- Combien de gamins sont là ? asks how many of the kids are at your location. It is really a different question but translated the same way to English.