On Wed night he stayed with the kids at my ex's house and apparently acted odd. He hid under the table and snarled when they tried to encourage him out. On Thursday when he came home he was lethargic and quiet. He hasn't eaten since Thursday am. He has pooped and has smelly tarts so probably has an upset tummy. In the car cage yesterday a friend's daughter went to tickle his head thru the cage like we always do and he snarled! Today he got between the fence and the play house and I squeezed down to try and help him out and he got very aggressive! I've just taken him to the vets who said he's medically ok.It sounds like he has definitely got issues with children now -I've seen it happen as well, a dog I own was puppy walked by the MOD by a family with four young children, and something clearly happened there as she now really dislikes children.
As he had an upset stomach on Thursday and was at your ex's house on Wednesday, I'd wonder if he was given (or stole, or something was accidentally dropped) something to eat that upset his stomach -that would easily explain that bit. He hid under the table and snarled -did he have anywhere else to hide where he could feel safe? You mentioned that at home he has a cage and also a dog flap. With the kids then trying to coax him out, it sounds to me like he was scared and felt cornered. There is nothing much more dangerous than a scared dog that is cornered, so you need to teach your kids that if the dog growls, they are to walk away and leave him alone -he is being good and warning them that he needs space. If he hides away, let him.
Growling in the cage in the car -sounds like the person who tried to touch him was also a child? He's now had bad experiences and the cage is a safe place but again he is cornered when it is shut and in the car, so he was again saying that he wanted to be left alone. If growling is ignored over and over he will have no other choice but to bite, so again best now to not let anyone touch him when he is in a cage. When he got stuck and you tried to help him, again it was fear -he was cornered again and now has bad experiences of this and simply did not realise you were only trying to help.
You DO need a good behaviourist, ideally one that can come to your house. But in the meantime, the main thing to do is give the dog more space, tell the kids to leave him alone and wait for him to approach them rather than the other way around. If needs be make use of more gates -you can get extra tall child/dog gates that a 4 year old will not be able to reach to open.