Regarding noise from luau cove, I personally do not think it is an issue. We stayed in Aotearoa which is closer to the luau. I very much enjoyed hearing the music on the balcony but with the slider closed it was silent. I would, however, highly suggest bringing a noise machine/fan to drown out the typical hotel noises if your son is a light sleeper. We take one everywhere we go because I hate hearing other guests being rude in the hallways or the constant door slamming at all hours of the night. You never know if you will have considerate neighbors or freaks so plan for the freaks and be suprised if they are normal
I would prefer Tahiti because of the larger rooms and the nice deserted beach--BUT--if you are concerned about noise, the Tahiti longhouse will get more horn noise as it is closer to the TTC ferry. Both Tahiti and Tuvalu will catch wind of the Poly/GF boat horns at the Poly boat doc. Again, bring a noise machine for sleeping peace.