"Blackberry" is a proper noun. Apostrophe is needed.

Blackberry´iniz var mı? (Do you have any Blackberry?)

Indeed, "sahip (owner)" can be used as well. It´s used with dative case.

Bir Blackberry´e sahip misin? (Do you have any Blackberry? (direct: Are you an owner to a Blackberry?))

But this way sounds very formal. (:

I hope these are OK?

Do your children study here?

Burada çocuklarınız çalışıyorlar mı?

Do your children study at Sandhills?

Sandhills´de çocuklarınız çalışıyorlar mı?

They are okay. But I want to point out a comon mistake. lar/ler is the personal suffix in "çalışıyorlar" and it means "they". In Turkish all personal suffixes are put at the end of the question word. But when it comes to this ler/lar suffix, I dont know why but people generally put it before the question word.And it is wrong. Let me give you an example.

Çalışıyor musun? Are you working? As you can see personal suffix "-sun" comes after question word "mu"? It would be so weird to say "Çalışırsın mı?"

Çalışıyor muyum? Am I working? Here personal suffix "-um" is put after the question word "mu" too. ANd it is also weird to say "çalışıyorum mu?"

So instead of saying "çalışıyorlar mı?" , we should prefer saying "çalışıyor mular?"

I got it from this website:

http://turkish.pgeorgalas.gr/ConjugationSetEn.asp

They only do this for they:

Question-Soru
çalışıyor muyum?
çalışıyor musun?
çalışıyor mu?
çalışıyor muyuz?
çalışıyor musunuz?
çalışıyorlar mı?

Is it wrong or both ways are possible?

Thanks!

So I went to the Döner today. I said to them "Sandhills´de çocuklarınız çalışıyorlar mı?"

They said çalismak was not appropriate in this case since I was referring to Children attending the school. Is it appropriate to say "nerede çalişiyor musun?"? for school? only for University?

Yeah they are right. I only checked your sentences gramatically and told you they were right. But  I have noticed now that semantically they are wrong. "To study" means "okumak" in those sentences.

Let me give you examples:

Do your children study in this school?

Çocuklarınız bu okulda mı okuyorlar?

Do you study at university?

Üniversitede mi okuyorsun?

Actually okumak means "to read" but we use it in the meaning of "to study" too.

If you want to ask someone if he is studying in the sense of doing his homeworks or reading a textbook then you should ask like this:

Do you study?

Ders mi çalışıyosun?

Note: I put these mi´s(question words) not at the end of the sentences but after what I want to emphasize.  Let me give you an example:

For example:

Do they study in Sandhill ?

Sandhill´de mi okuyorlar?

I want to learn if they study in Sandhill. So I emphasize Sandhill putting the question word after it. If I put it after the verb, then I would be emphasizing the verb.