Is it safe to bring young children (3/4 year olds) to this facility, i.e. are things organized so that there's nothing dangerous that could cause tripping, falling, cuts...?

Are your children well behaved? Will they stay right by your side at all times? If the answer is no, then I wouldn't suggest you bring them. While SCRAP is organised by material type, it is a warehouse that people dig through often. There are hazards if you are not aware of your surroundings. That bin of buttons? Don't plunge your hand in because there are likely pins in there too. The aisle in the back with random bits of wood, metal, and glass? Beware of nails, staples, sharp edges, etc. I am not trying to scare you away. This place is amazing and I've brought my daughter with me plenty of times. But she was old enough to listen when I told her not to do something.

Children must remain supervised