Yes, at least as far as your child's eyes are concerned. There's no evidence that watching something in 3D damages a child's vision. In fact, watching 3D media is pretty much the same as watching something in real life. That's why the American Academy of Ophthalmology says watching something in 3D is unlikely to harm your child's eyes.
For regular sight, your child's brain builds a three-dimensional concept of the world using slightly different images from each eye. At a 3D movie, the special glasses create a three-dimensional effect by picking distinct images for the right and left eye. With 3D video games, the interplay between the placement of the images on the screen and your child's head positioning work together to provide specific images for each eye.
Lately, video game designers and other 3D device makers have warned against letting children younger than 6 years old use their products for fear of possible damage to their developing vision. However, there isn't any conclusive evidence – or any persuasive theories – that this is a valid concern.
Contrary to the warnings, your child's eye development is largely complete by age 3, long before extended viewing of either 3D movies or video games is likely to become routine. And in any case, 3D viewing is remarkably similar to how a child sees in real-life environments from birth to age 3.
On the other hand, eye fatigue and nausea from motion sickness can be a problem, especially when your child is mature enough to watch a movie in a theater.
Starting with the opening credits, filmmakers throw a lot at the audience. The constant motion, exaggerated differences between what the two eyes see, dim lighting, and often rapid oncoming movement and focus changes can be hard for some viewers to handle. If your child feels queasy, tell him to close his eyes for a few minutes or try sitting either closer or farther away from the screen.
Some people are much more likely than others to feel eye fatigue and motion sickness. If your child has problems with eye misalignment, such as strabismus, he's more likely to feel tired, headachy, or nauseated when watching something in 3D.