"Il y a combien de gamines ?"
Translation:How many kids are there?
From a Canadian perspective "youngster" is quite old-fashioned, and I rarely ever hear people say it (at least under the age of 70), but it should be worth considering if it's used more commonly in other English-speaking countries. Even Merriam-Webster's learner's dictionary refers to it as old-fashioned: http://www.learnersdictionary.com/definition/youngster
There are already other words in French that mean a youngster - un(e) jeune, un mineur, un enfant. Un(e) gamin(e) means a kid, it does not matter if you would not use this word in English; you do not have to use it in French either, you just need to demonstrate that you can understand it when someone else uses it!
From a "type what you hear" exercise, you must type it exactly as given. There is a pronunciation difference between "gamin" (nasal "n") and "gamine" ("n" pronounced). That's not all. Try listening to "gamin, gamine" on Google Translate. You will hear the difference very clearly.
That would be similar to saying "Are there how many kids?", in English.
That sentence structure doesn't work when you have a question word like "combien", "où" etc: the question word has to come before the inverted verb. So you could say:
Combien de gamines y a-t-il
See this Yahoo answer