This was my thought exactly.
I drive by that lake on Hwy 20 all the time, and it doesn't seem like the type of place 2 kids could drown in easily. there's no current or anything like you get in a river.
Given that it's two of them, and that there is no trace found, I think the most likely scenario is actually a run-away, or abduction by someone they knew, that they went with willingly.

