Is the ocarina a good instrument for young children?
While children certainly can learn to play the ocarina with a good approach, I cannot say that the ocarina is universally a good instrument for young children as there are many factors at play. Teaching music is a complex affair because everyone learns differently, so no single approach is universal.
The ocarina's simple appearance can be deceptive, as playing the instrument well is quite technically involved. Wind instruments often look easier than they are because many playing techniques happen inside the body. For example, the tongue must be used to create articulation. This can be unintuitive to children who generally 'puff' into wind instruments, creating a very indefinite sound.
The ocarina's pitch is also highly unstable, and requires a lot of skill in breath control to play in tune. If a child happens to have absolute pitch, or has developed relative pitch from a prior instrument, this skill will develop organically. However, lacking these skills and without guidance, it is easy for a child to play wildly out of tune without their awareness, creating a false sense of ability.
Finally, playing the ocarina musically requires the learner to develop an awareness of musicality. It is pretty easy to 'fake' playing music on any instrument by visually showing someone how to play the notes, but such performances generally aren't very musical. Adding interest comes down to the application of ornamentation and subtle changes to rhythm, details that are best learned by listening to and studying many performances.
None of these are impossible to learn but they do need to be taught. Don't think that you can lazily give an ocarina to a child and they will learn to play it with no guidance. Exposure to instruments is a good first step, and generally playing around with one will allow a child to learn how it works, but this is not enough. Personally, I never grasped music as a child largely because I lacked this guidance, even though I was surrounded by instruments.
Do be aware that ocarinas are loud and have a very piercing tone, so they can be fatiguing to listen to for prolonged periods. Learning an instrument absolutely demands repetitive practice. Thus, if you are a parent and your child takes to the instrument, you'll hear this sound for years. You have to be willing to accept this.
Whether or not a child successfully learns any instrument depends a great deal on approach, something that I think is often lacking. The page 'Teaching music to children' covers my thoughts on that subject.