Picnic/Standard/Expert is more like Picnic/Casual/Normal, to be honest. From Standard to Expert, you take 66% more damage, and deal 33% less. Also, the battle retry you get on Standard should pretty much prevent game overs from occurring unless you're forgetting to bring threads and get into a really bad situation.
Picnic goes the other way around and has you take 66% less damage and deal 33% more, and your heals are 33% or 66% stronger (don't remember), so enemies do piddly squat. Your thread doesn't go away on use, and you get infinite battle retries, so there's virtually no risk of something unpleasant happening in this mode. I suppose for little kids, this could be a good difficulty...?