Keep an eye on the 2 black ones. Their legs are pretty bowed.
There is generally a nutritional deficiency that causes this.
If they don't start straightening out you will want to see your vet.
Copper sometimes but generally you don't give copper to a kid. I believe someone else had a similar issue and it was rickets.
There again it could just be the picture and that they were lifting their leg to walk and it just was at an odd angle.