Theoretically, it is Ok for 5 years old kids see 3D movies. However, being an adult, you may also feel uncomfortable after seeing 3D movies for too long. I believe you have such experience that when you watch too long 3D movies, your eyes feel strain or dry, even shed tears, and headache. 3D movies required the eyes to converge in front or beyond the screen. So parents you had better pay attentions to your kid's reaction. I suggest that watching 3D movies do not exceed one hour and have a rest. Second, when watching 3D movies, trying to sit at a distance from the screen, not too close.
As a matter of fact, seeing 3D movies is quite bad to 5 year old kids' eyes to some extents. As we know, while watching 3D movies, our eyes have to focus on the physical screen and the 3D content. That is to say, our 3D glasses filter light that everything has double images. We should accommodate to the distance of the screen because that's where the light comes from. What it means is this will exactly increase the burden to our eyes. In this way, your eyes can easily get tired and then you may even suffer blurry vision, the rather that kids who have frail eyes. Therefore, it's better for kids not see 3D movies in some ways.
Oh, your question is very good. So, from what I know, children's eyes are very fragile and are still nurturing, which means that their eyes should be well protected from getting damaged by 3D movies or some electronic devices. 3D movies, would make our eyes strained and uncomfortable, let alone children's eyes. Now I think you know what to do! Also, try to pay attention to what they eat and their rest. and use of eyes.