Precisely because of that--it was accidental. If they did sedate Maddie and she had a reaction of some sort, and died, it would most probably mean the loss of their licenses to practice medicine. They might very well lose custody of their other two children as well. You or I might be able to plead that we didn't know that the little bit extra of cold medicine or adult medicine would actually kill a child, but the McCanns would have no such excuse.
Faced with losing their other children, loss of livelihood, and maybe even prison time, they might very well rationalize covering things up. After all, doctors make mistakes in judgement every day that result in people's deaths. It's something every doctor has to learn to live with in some way or they can't practice medicine.
As well, there seems to be about one child every year or two in Houston that dies from a reaction to anesthesia or some problem with being sedated, while in a dentist's office. It's awfully hard to correct a mistake with sedating someone without a crash cart and all that other "stuff."
The immediate elevation of Maddie to poster child for all missing children was always a bit concerning to me. The McCanns were so quick to begin to pointing out ways the system could be improved and commenting on the number of children missing every year, etc. Even the balloon launches and pipe concerts seemed more memorial in nature. Possibly they decided that since it was an accident, this way the cover up would actually be a good thing because it would help missing children. See, they would be making tragedy into something good for other children.
Just a thought. I'd love to be wrong on this and have Maddie show up alive and someone claim that nice big reward. Oddly enough, with all this talk of accidental death, wouldn't now be a good time for someone to step forward and get that reward, before it's possibly taken back? There doesn't seem to be even a hint of that happening--another reason I don't feel hopeful about Madeleine's life.