I've responded about the harassment in a separate post as its a separate issue.
postin stuff on FB about us, saying things as we walks past calling my hubby a kiddy molster
What are they saying about you on Facebook? If they are making the above accusation, this is serious libel and you can take them to court. You will need to take professional legal advice on this. Screenshot every instance, also note/screenshot if you can where it's being posted - Is it on their personal profile, or in a local community group? Something posted to a private profile with two friends would be seen as less damaging than something posted to a public local community group with 10,000 members, for example, so this is very important information.
If the accusations are being made verbally, this is slander and more difficult to prove without evidence.
Both libel and slander are civil matters, and very expensive/stressful/time consuming to prosecute.
However, if these accusations are being shouted at you in the street, in public, along with other unwanted behavior that is genuinely making you feel threatened/distressed, this likely qualifies as harassment, which is a criminal matter. It must have happened on more than one occasion and be obviously unwanted (eg, you've told them to stop...although with this particular accusation the bar for unwanted is almost certainly met by default).
You should evidence as much as you can, and report every instance to the police in the first instance (non-emergency, unless there is an immediate threat) and any LA/neighborhood safety teams in your area.
Remember though, a letter through your door asking you to provide/delete footage of a person taken outside your boundary is not harassment (unless it contains threatening/abusive language, eg "If you don't send me that footage by tomorrow I'm going to do X, Y or Z") and you must comply unless you have a legitimate reason not to.
her eldest sits waching our house.
From where? This is unlikely to be considered actionable unless he's trespassing on your property to do so, using equipment to peep through windows, recording, behaving in a threatening/intimidating manner etc.
If they've made legitimate requests to you to delete footage of them and you haven't complied, they may feel harassed and be 'keeping an eye on you' as an alternative to installing their own camera.
Plus how do you know he's doing this? You're effectively watching him, watching you.
he makes.out like he is going to pull over an start something when Im out walking the kids to school.
What is he doing exactly? Driving past, looking at you, or something more?
If it's the former, ignore him and walk away. He likely has a legitimate reason to be driving near his house and while you're embroiled in this 'feud' he's obviously going to be giving you the side-eye and cursing you under his breath. He probably thinks you're doing the same to him.
However, if you have reason to believe that he's specifically driving that way to follow/intimidate you, or he's verbally threatening/insulting you, or driving in a way that you feel is putting you in danger, record it, get to somewhere you feel safe as quickly as possible (even if it's knocking on the door of the nearest house) and call the police.