I find it important to draw a distinction between "idlers", (dwarves currently without a job) and "party animals", (dwarves who routinely do nothing except aggregate in the dining hall, forming dangerous friendships. Dwarves don't let dwarves make friends.)
Idlers are fine. Party animals destroy fortresses.
Keeping the former from becoming the latter, especially with what are essentially proletariots thrown in (fish cleaners, millers, etc-- that only work "ocasionally") can be quite challenging.
Burrows help, since you can keep a single "prole" in each burrow, and keep those idle hands from being the devil's plaything that way, but they aren't perfect either. (Some have to be outside burrow restrictions, and the can make the dreaded friendship tree branch between burrows. This is because you need some dwarves to move goods between burrows, or from outside the fortress, to the inside. Minecarts only do so much.)
Children are the bane of my gameplay experience, because they are almost always, without exception, party animals, and undermine the fortress' morale. Children that aren't locked up into maturation booths are 5th columnists for the goblin army, and need to be contained until they mature enough to understand that they need to work, and need to stay in their burrows.
Idlers are fine. Party animals cannot be permitted. Parties are only allowed as needed, to combat psychotic episodes after seeing the undead or something; and then only as private parties.