I'm afraid that after another 3 years (on and off) of reading this forum, I look back at the opening post saying the stepmother took a 'nap' and Diamond and King disappeared with different eyes than I did back then. Yes, things can happen when you're asleep, but then...
...to be honest that was the only explanation I could think of back then for why there wasn't more apparent LE investigation going on for a toddler and a young woman whose age belies the reality that she is also effectively a child.
When you put these two things together, what do you get? I hope that's not the case, but when these seem to be the two main pieces of information regarding this case, the ones that really stand out as 'facts'. I hate saying that. Maybe they did wander off and searches just weren't thorough enough? I can't recall what kind of area they lived in, but if it's not particularly rural it seem unlikely.