DW was specifically asked in the interview with Mrs. Cher if SW had been feeling unwell. He said she had not been and that he didn't know where that came from. Of course, he was not with her every minute, so....
I agree it is possible being in the horse pond was an unhealthy experience. The water level of that thing will vary. All that matters to this is the depth on 6/15 which we can get some idea of from the video, comparing it to her height. It is possible that someone has a better idea of exactly what was going on with it. It is controlled by TVA.
But, LE seems to think SW went missing from the Wells'property. All the searches are near it.
They used to make over the counter medication that had a coating that tasted good. Not candy good, but good enough to make them easier to swallow. I have not encountered this in longer than SW has been alive. So, i am doubtful SW could have been given any medication that tasted like candy. Even back when they made these, they didn't smell like candy.
Also Sudafed does not cause drowsiness.
https://www.rxlist.com/sudafed-side-effects-drug-center.htm
What does cause drowsiness is Benedryl, an antihistamine. It is probably more common than Sudafed, but it doesn't taste like candy although it is often pink.
However, unless someone was going to put SW in a duffle bag or something (temporarily), they would probably do better to lure her away than drug her because she is only five and therefore probably not that difficult to trick. They could just tell her that her daddy is down by the mailbox with a surprise. As soon as she figures out that he is not, grab her. Just speculate on how someone could do this.
I believe if any random person had been on the property actually talking to CB and CH they would have said something. It doesn't take an elaborate ruse to understand how CB and CH did not see SW possibly leave.
Line of sight is blocked on that property by various things they have in their yard. Also it is possible CB and CH went into the camper for a few minutes.