I am really sorry for what happened to the mother and the child.

But I think, they should booked while they are boarding the flight, the boarding attendant would be so helpfull regarding this, coz they the one who arrange the seating by giving out the seat numbers.

If it is not booked, the air hostesses then should persuase those front seat passengers to exchange the seat, there is 2 possibilities :

1) the air hostess didnt do their job well, (then you can blame them!)
2) or the other passengers just so ignorance to move for a sick child.

Btw, for relieve, go complain to the air arabia office accordingly.