Birkaç Türkçe çocuk kitapları alabilirim nereden?
Where can I buy some Turkish children's books from?

I think you should switch the last two words' places

And the title should be kitabı

i think it should be kitaplar çocukları

Cause Çocuk here is the Sıfat , Zannettim

Yes, it is "belirtisiz isim tamlaması" (undefined noun modification).

There is a problem with "birkaç ... -lar".

well,i guess you are right here,both are nouns,but from my point and i don't know if it is right or not
The books are the wide choice here,and you are spescifically want a certain kind which is children books,so i guess it should be like this

Kitaplar çucukları

pls,correct me if im wrong