We recently went to a local park where there was a group of about 6/8 primary school aged children. Rhey asked if the could aproach him while he was tied to a fence, 'yes, is ok to stroke but may jump around'. He is a big dog.
The kids come up to him and start jumoing around his head - He lunged and growled.
Sorry this happened, I imagine the situation was at least unnerving for you and the dog (who knows with the kids?). Good on you for seeking feedback and advise, that's important when figuring out what might be going on.
That said, the first thing that jumped out to me is the portion that I bolded above. Unless it was some kind of emergency, I'd
never tie a dog in a public location, much less in a park with unpredictable and likely uneducated children running around. I get that you may have thought to keep your hands free so that you could deal with your own children (if present), but that decision set up a potentially dangerous scenario. How so? Well, first you've taken what sounds like a new-to-you dog to what sounds like a busy public area. You don't know how comfortable he is/is not with strange children and you don't know whether these children know how to approach an unknown dog (which it sounds like none of them did). Further, by tying him to the fence his response options were reduced to two (submit or protest). The outcome could have been much worse. Finally, if a child or adult is approaching my dog in a provocative or problematic way, I don't ask them to stop, I TELL them to stop and I'm prepared to back it up as needed (e.g., standing between the dog and the approaching children/adults, raising my voice). I don't stop until they do and are standing well back. My first responsibility is to protect my dog from clueless people, big ones and little ones. (Your priorities may have been different if your own children were with you). I also want my dog to learn that s/he doesn't have to intervene, because I will take care of things. It's generally safer and less stressful for all concerned.
Bottomline to me is that it sounds like you haven't had this dog for very long and don't know him very well. That's not a flaw, nor even a problem necessarily. But you've got to be more mindful of how you manage things until you do know him better. I'd change how I'd manage things in the household and in public. Here are some quick suggestions for how you might achieve that. First, start basic obedience training now, particularly leash manners. A trainer would be helpful, but you can do much of this on your own. Second, consider the two week shutdown previously suggested. He needs to learn the rules of your house and, eventually, the rules in public. It's a nice to think that we can just fold a dog into our households with lots of love and without much effort, but things rarely turn out well that way. More care and management sounds necessary. Third, you've got wee ones at home. Even if he's "good" with them, any excited or sudden movement on his part could injure them badly. So, I wouldn't allow the dog and the children to be together unrestricted (e.g., either crate the dog or keep him on a leash until you're
certain that they all know how to behave around one another). And, you're going to have to teach your children how to behave around this dog, as well.
Fourth, gradually introduce leash manners so that you can go on walkies --- without your children. Once he's well on his way to good leash manners, walk to the park but stay outside and observe the goings-on. You're trying to teach him that the park isn't a big deal, children will not rush him and if they do, you'll protect him. When he's calm about that, trying walking through the park (without your children!) --- keeping strange children, adults and dogs away from you. When he appears rock solid about all of this, then you can consider incorporating your children into walkies.
What I'm describing is a lot of work, especially if there are young children in the house. But the benefits are priceless and you establish a close bond with your dog, to boot.
ETA. In a later post, you mentioned that he's "grumpy." Well maybe he is and maybe he isn't, but you should
pay attention to that. He's communicating something, whether it's discomfort, fear or merely his way of "talking" it's difficult to tell. I'd take steps now to make sure that he's got a safe space in the house (i.e., a crate) from which to observe the goings-on. This is where a trainer would be particularly helpful. S/he can come to your home, observe your interactions (which we can't) and suggest better/different strategies for managing things.