"Op you havnt over reacted, maybe a little with the smoking thing but not the rest!"
This is what I was getting at earlier, when I said that the problems with neighbours often start because everyone has a different idea of what can be classed as quiet enjoyment and nobody thinks they are as bad as the people at the other side of the fence.
Nobody has agreed that the stone throwing was okay, but people have picked up on the OP changing her story by saying first that the boys were throwing stones at each other and then that they were throwing stones at her baby. They weren't throwing stones at the baby, although the end result could still have been very serious if one had hit the baby. Nobody is disputing that but the OP shouldn't change the story to make it sound worse than it is.
The OP complained, the mother lined the boys up and had them apologise. And nothing else has been thrown over the fence since then. So here, the parents have effectively controlled their sons and their behaviour, something the OP says they can't do, yet they clearly have.
You say the OP has overreacted at the smoke in her house but plenty of other people have said that would be the bit that bothered them too, and be the main reason they would have for complaining.
And people are also divided over the younger boys showing their penises to the OP's DD, who probably couldn't even see them. The OP has changed her story there as well, from saying the boys called her DD's name to saying they shouted abuse at her.
And she's also said that her daughter would cry at just the name calling, so she's not even sure if her DD saw the penises. She could have just been crying because two boys she doesn't like, boys she will have realised her mother doesn't like, one of which hit her (and again nobody has said that was okay) called her name.
Children, boys and girls, often do go through these stages of being interested in their genitals. The OP's question, her very reason for starting this AIBU was to ask if at five years old this boy is too old and should know better. And the resounding answer was no. Boys and girls of this age are still discovering themselves, their similarities and differences.
The eight year old didn't join in. He's clearly reached the age when he feels too old to join in with the younger boys and he's being raised by the same parents who the OP feels can't control the younger two. The five year old will get there too and it's not like they are there with their penis's out every time the OP sets foot in the garden. Like the stone throwing, it's a one off annoyance that the parents have probably dealt with, just not to the OP's satisfaction since she wants then to be in ''huge trouble" with "punishments".
Yes it has annoyed the OP but it wouldn't, at that very young age, annoy everybody. And I do believe the boys parents should have a quiet word and tell them not to annoy the neighbours by using the trampoline to see over the fence, either to call out or to try and show their willies to her 
But all this nonsense about "exposing themselves" and "what would happen if a little girl played with herself while the neighbours watched" is just making an annoying but innocent situation worse. That sort of language and all this asking "but when will they stop - when they are fifty?" hysteria is making the boys out to be budding sex offenders when at this age they are not even capable of 'playing with themselves' in the way most adults understand it.
I suspect what has annoyed the OP more than the penis's is the fact that the mother has finally had enough of her complaining about everything and told her where to go.
Which I cannot blame her for because if she went there talking about the boys "exposing themselves" to her daughters the mother probably heard the same sexualised language as most people here and found it just as ridiculous. And she's probably well aware that the OP feels they are not "her sort of people" because you can't hide an attitude like that.
I'm sure she still had a word with the boys after the OP had gone, because she has done so before about the stone. But the OP is pissed off because she feels that their apologies are not sincere enough and a word is not as good as huge trouble and punishments.