I’m going to take the low road and suggest doing what you do with any other vermin attacking gardens… pellet gun.
That was a really funny rant. I know you’re annoyed but I do love the use of the word “heathen”. My mother used to use that word to describe my sister and me when we were kids.
Anyhow, I agree with planting poison ivy. I think that’s a great idea and would work without looking like you 're obviously taking revenge.
Bribe the little—darlings.
This is the only thing that will work. If you become their cookie fairy, they won’t WANT to mess up your stuff. If you go over their heads to a parent or a condo board or whatever, they’ll just do it more sneakily. It will get worse.
Take the Mother to Small claims court. Build a case document the damage they’ve done and the sue her for damages.
If you’ve planted your flowers in a common area, there may not be much you can do other than ask them politely not to step on them. Perhaps if you planted them in pots, it would help.
Pots with razor sharp shards of glass, to discourage them from digging around.
It sounds like you’re the bitchy, mean neighbor lady that all the kids hate.
Yes op is a horrible person for not wanting them to trash her garden.
I’m a dude. Unfortunately my “garden” is a little square of dirt that is under my stairs. So yes it technically is a common area which is why efforts or actions beyond the chemical warfare idea and bribery won’t really work. But any landscaping in this particular common area is up to the owner/resident of my particular unit. The lady whose kids piss me off has her own common area her kids can stomp around on and it would be super because the landscaping there consists of river rock and mud.
Its just frustrating because its not in the middle of a lawn or anything, so there really is NO reason beyond being pests for the kids to go under my stair area. Among all the other stuff of course.
As for being “bitchy”, all I have to say is “get bent”.
I suggest talking to the older kid alone. Tell him that you view him as the man of the house, and as someone who has control over his younger siblings. Then let him know, in no uncertain terms, that you are going to hold HIM responsible for ANY damage caused by any of the heathens. Of course, you’ll have to leave it vague, but make it so the physical threat link is created in his head. If he does a good job, find some way to reward him.
Good luck.
I would do what magellan suggested, but spin it positive instead of threatening. Say, “I see that you’re the man of your house, and you’re in charge of your siblings. If I can trust you to keep them out of my garden, I’d be happy to buy you guys some ice cream pops a couple of times a month, or something. What do you say?”
It might work. Or it might make you seem like a creepy neighbor guy, not sure. But since nothing else is working, offer of reward might be the ticket.
I vote for chemical warfare.
There’s no reason you can’t plant whatever you like in your own garden…so if you plant something with unpleasant side-effects…
If their mother won’t discipline them, your garden may have to.
If you do plant some attack plants I recommend stinging nettles. In addition to being highly nutritious folk remidies use them for everything from blood thinners, bleeding control, prostate problems, arthritis, lots of good stuff.
They grow easy like weeds, and the best part. They give you horrid welts if you touch them. I hated those thing growing up.
Ths is true. I am a single mom and my kids did not behave like heathens because of it.
I am not staying they were perfect but when they did get in trouble, which was not often, I made them own up to it.
We had a heathen kid down the street when my kids were growing up and that kid was always in trouble. Breaking windows in cars and houses. He smashed our pumpkins one year at Halloween for no reason. The neighbor across the street seen him do it and told us. When we went to his grandmother, his mom was never around, she blew it off like it was just something that kids do.
Pissed me off because if it was my kids I would have made them not only apologize but made them work at extra chores or something to at least pay for the pumpkins.
I understand that you cannot always keep on eye on your kdis 24/7 but when they do get into shit make them own up to it. It starts with ripping out plants and continues to ripping off cars.
It was halloween.
Is there any way to fence that area off completely?
As for your last line I think the phrase you were looking for is “Get off my lawn!”. 
I’d have to agree with taking the more positive approach as suggested by Rubystreak. But I would still include in the equation the notion of pain coming his way should he not be sufficiently enticed by the carrot.
Less horrible than nettles, but still give me itchy bumps is Borage. Pretty little flowers, weed-like growth and selfseeding for the following year.
Feed them to a woodchipper.