My views on the subject are, anything that can allow a large amount of the community to meet up for some form of event that would not cost a huge sum of money is always a good thing. However a venue to hold such an event is likely to be expensive, assuming we get a fair few people and if we don't then is it going to be anything special other than a few people meeting up, it that's all it is then that's been happening for ages.
If it is going to be a big convention style then it's going to have to have an entry cost to cover the cost of the venue, for people to pay an entry cost there needs to be some big selling points. Obviously meeting people from the forums is one such point, but with time then you can go out and meet anyone willing to see you without an entry cost. Things like some widescreen TVs or projectors showing Knightmare series, or the VR pilot sequence or even the RPG would be selling points. Perhaps some Knightmare merchandise, though there isn't a lot of it.
Tables with the boardgame would be good, if a few people could bring it & have several games going. Or using a roleplay system such as Dungeons & Dragons then a Knightmare scenario could be made for people to play. Possibly even some Live Action Roleplaying All of these would take some organising of course but it'd be worth it for a big convention & if it's successful it could become a yearly event.
Obviously the manpower required to organise this would be a fair amount, a suitable venue would need to be found & booked and some method of buying tickets arranged. With pre-registration being important so the organiseres can get an idea of the numbers they're likely to have on their hand. If tickets could be issued by post then that would avoid those who pre-register from having to queue for ages.
It will be difficult, with Knightmare fans dotted around everywhere, to find a venue that suits everyone perfectly but I am sure that any big venue would be close to rail & coach links. Providing the travel & entry costs are not too high then I would say the idea has real promise, with someone with good skills working on it then it could go far. As Kieran has volunteered then I think there's no need to worry about it being in good hands
As an afterthought, the idea of giving any profits to a childrens charity sounds like a wonderful idea. I apologise for ranting on so much also, just wanted to detail my views & ideas all at once rather than disjointedly.