If the cabin is at zero you need to do a lot more to keep them warm. It is far too low - they should not really below 15 degrees.
When you say the ones on tile, what do you mean by this?
I assume you don’t have electricity in the cabin?
At the very least you would be well advised to fully insulate the entire cabin if they free roam within it.
Proper celotex insulation and then plywood line the whole cabin is the most effective insulation but the silver foil insulation is also good.
My shed insulation alone is keeping the shed approximately 8 degrees warmer than outside, in addition, the fact my piggies are in hutches, the hutches themselves are also insulated and lots of hay and heatpads means despite it being -3 outside, my hutches are currently sitting around 12 degrees. So you can see if makes a world of difference.
For now and while you can arrange insulation, lots and lots of hay stuffed into hides will give them somewhere warmer to sit, but heatpads really are essential for outdoor/shed dwelling piggies - one pad per piggy.
How did you come to have pregnant sows?
How old are the babies? If they are very young then they (and their mother) do need to be brought indoors. Cold is particularly dangerous to youngsters