Duct tape their PJs
The first official hack that employs duct tape! (To be fair, Laura BC did mention duct tape in her comment on Low-tech babyproofing hacks.)
Charles tells us:
When our kids were still in diapers and wearing footed PJs with zippers - and they got old enough to think it was funny to unzip and take the diaper off - we'd put a strip of duct tape across their chests so they couldn't unzip.