I was looking for black plastic picture frames the other day for my living room. My kids like to pick up various photos in the house and look at them and comment on them.
These are quite durable and can withstand water and other stuff like play dough children often put on things in the house. It can be washed and cleaned as desire. They are light weight and can be handled by kids easily.
The first criterion was obviously the color, as I wanted to match the color in my living room. The material had to be synthetic / artificial. Weight was also a consideration as I wanted the kids to be able to pick and see the photos if they want. They have to reasonably unbreakable.