Ahsanallahu ilaikum ustadh, since women are required to wear the full hijab, does this means they are not allowed to wear a sling to carry their babies on their backs. This practice is very customary where we live. Does the same ruling also apply when wearing thick jackets on top of their clothing?

The condition is that what she wears does not show the size and shape of her body, so a jacket is no problem at all, it conceals more and not less, but she puts her khimar on top of it from her head to under her chest. A sling is judged by the same measure, if the straps press the covering so that the chest and shoulders are outlined then she should find another way to carry, and if they do not then there is nothing on her.