Problem with the children growth

Perhan's sister does grow up. In the first half of the movie, we see her only as an ill child bound to bed, constantly weeping. At the end, her hair changed color, grew longer, and if you look closely her facial structure, she does look older. Children who suffer such physical condition never fully develop, so the looks can deceive.

As for Ahmed's child, I think it was intentional. Kusturica is famous with details, and he could not have made such a big mistake. You see, one of major subject of this movie is family, or, to be precise, the lack of one. Everyone in this movie wants to belong somewhere and to be loved, but their families are shattered, or simply cannot exist. The great part of understanding this comes from understanding gypsy culture. They are nomadic people, who loves freedom more than anything, but they are like leaves on the wind. Some of them who can afford decent life still live in the slums and roam around, that is their nature. Children are universally a metaphor for innocence, and innocence in this movie is easily sold. You remember how badly children are treated in this moveie? The are not people, just stuff, a means to en end. So, Ahmed's son can represent both innocence and a mythic structure of family in a gypsy culture ( remember that story Ahmed told, about a man who has 11 children with his wife, and two with his lover, and he had to sell those two to avoid shame, that shows how contradictional their set of values can be ), and that combined makes a greater metaphor about an entire gypsy race. By leaving his grandmother and Azra, Perhan slowly loses his innocence, and becomes sold out, trusting Ahmed, his only father figure, but his conscience is always troubled by his dreams and visions from the past. Ahmed's son helps develop this with his bunny toy ( another metaphor for innocence and vulnerability ), which plays ''Tamo daleko'', an old Serbian melody about a soldier who craves for his long lost home and family. At the end, that child becomes an orphan like all the other children, meaning that those people will always be alone and shattered, no matter what happens, no matter if their parents are beggars, criminals or honest people. He cries, but not for his father, but for entire race, because those people need to have a family, and family needs a central, father figure, and the great misery is that central figure needs to be someone like Ahmed. It even goes further, it is not by accident that this the only child that does not grow up normally, he is the son of criminal, and petty crime like that in the movie is major part of many gypsy communities, and the main reason of the poor state of their lives.

I hope I cleared you a bit for you. I think that this can be understood even better with that great final scene with Perhan's uncle and Crucifix. It connects with child metaphor perfectly.