The general implication from watching the whole program - or at least the impression I was left with - was a genetic defect affecting the cerebellum (and therefore their balance), caused by inbreeding and affecting the entire family, compounded by their mother simply not having time to teach them to walk properly.
Footage was shown of the mother using the exact same stance as the quadrupeds when climbing to her feet - IE: on all fours for stability - and given that her husband was her grandmother's brothers son (Cousin, I'd guess?) then chances are the damaged gene affecting their balance was a recessive gene in their father too, and therefore reinforced in the children.
It was quite visibly clear the kids were brain damaged, all 7 of the worst affected (The quadrupeds and the older son with balance problems) were born in the space of 5 years, and it appeared all of the quadrupeds were using what looked very similar to a type of crawling babies do - I forget the name.
As mentioned on the program and as I said to myself several times before the program did - if children with that type of balance difficulty and level of brain damage had been born in a less poverty-striken society than they seemed to be living in, chances are they'd all have been in wheelchairs or recieved physiotherapy from a young age.
The program ended off by revisiting them a few months after the team left. On their original departure they'd arranged for physiotherapy-style parrellel bars to be installed at the farm. Most of the children were walking upright in one fashion or another - including Hussain (The worst-case one a Turkish physiotherapist said was likely to never walk upright).
As someone else said tho - was more of an over-dramatised, far too in-depth analysis of their condition than a documentary.