The great difference between the truth and a lie is that the truth is usually simple and a lie is invariably complicated.
"We went out to dinner leaving the kids most nights and we came back and they were still there" is really simple.
As is, unfortunately, "We went out one night leaving the kids but one of them got out of the apartment and fell off the balcony". Or, "We went out one night leaving the kids and we got back and one of them had had a serious accident". Or, "We went out one night leaving the kids and we got back and one of them had got out and been run over by a car."
The alternative is somewhat more complicated: "We went out one night leaving the kids, not realising that we'd been watched by a predatory paedophile for several days, who we hadn't noticed, despite the resort being quiet and despite the fact that we felt it was safe to leave the kids, who targeted Madeleine rather than the other kids because she was special, who got into the apartment between our checks, despite the fact we'd doubled the frequency from the night before following the crying incident, which we can't recall happening, avoiding being noticed by Gerry, who was talking to Jez, on the other side of the road to that claimed by Jez and Jane, who wasn't seen by Jez or Gerry anyway, just before she saw the abductor, who must have gone out through the window, despite no-one noticing it open and without leaving any marks, because the door slammed and the curtains whooshed when Kate went in, and it's odd about those dogs, but Kate had handled dead bodies when she went to work in her holiday pants and took the cuddle cat, and anyway the twins' sandals were in the boot of the car with the dirty nappies and the rotting meat, but we're totally confident in each other's innocence and our legal and PR team are too."