Are their eyes open yet? Once their eyes are open you know they are fully developed and it's a lot safer and a lot less likely to cause damage if you're careful. The filter is a good idea, as is using a zoom.
The best, of course, would be to take them when the flash isn't needed, in daylight. And if their eyes are open, as long as Mom is okay with it, you can take them out for short times and play with them. Just be careful not to pick them up too high from the ground, they can get scared from the height. Unless necessary, you''re best off by sitting by them and doing your thing close to the ground, and to their height.