Show lasts 45 min ( two performances daily). If you kid is sensitive to loud sounds, bright images, pyrotechnical effects etc - I would recommend to avoid Songs of the Sea.
My own impression:
1/ Show content is remarkably silly. Script writer is not worthy of Oscar, LOL. The whole idea is ridiculous and the story does not worth the cost of entrance tickets.
2/ There is no seat allocation - first come, first seat.
3/ Visual effects are so-so, Pyrotechnics OK. Fountain performance is what saves the show from total disgrace, but its full grace you can enjoy during the last 3 min or so.
4/ I saw SO MANY disappointed faces leaving at the end, I just can not resist to share the common mood. My face was sour too, I suppose.
5/ Very disappointing experience IMO. I would not recommend it, especially if you have a small kid on-board and unsure yourself about the suitability of this action to your child.