Someone asked me about this and I did some research. Just so the information is visible to everyone:

http://tasvideos.org/forum/viewtopic.php?p=498268#498268

Basicly, you have to pass 13 days after finding the kid, then set off with the big boat. Ending 5 will then be played regardless of the affection stat, even if it's above 240 (which triggers Ending 8 normally).

Treating the other kid bad has nothing to do with Ending 5. It's all about passing time.

Also potentially worth noting is that the timer for "other kid dies at sea" seems to specifically start when you insert the sixth gemstone - that address seems to increment before you find the other kid, but it resets to 0 as soon as you slot in the sixth stone and go to check on the other kid before going for the moon gem.

I think speedrun-wise we'd always wind up slotting all the gems on the day the other kid is met, but it felt worth maybe adding that distinction for anyone who might stumble across this looking for information on casual playthroughs :P