Looks like you are off to a good start. However, I hate to be the bearer of bad news. 4-6 hours is not that long. Let me give an example. I currently Co-GM a Dresden Files game, and we are treating it like a TV show with "episodes" I run an episode, he runs and episode etc. I attempt to plan an episode as a single session. Even keeping the adventure simple and streamlined, role playing, interesting conflicts, tangents, and zany antics usually stretch the episode into a 3 sessions of 4-6 hours. And that is when the setting is already defined, the characters are already created, and the players know the rules.
My suggestion is before the session, prep things to insure a smooth session:
1) Completely generate characters (Kids on the Island) Make sure to include a paragraph or two of descriptive text, keep it vague, but help guide the new player to understanding the character motivations.
2) Completely generate the environment (Island, scenes, danagers, clues, demon stats, location of boat off the island, etc.)
3) Pre-write some descriptive "box text" compels for each of the characters via an aspect, concept, or trouble to be in the location you choose.
4) Pre-write a descriptive "box text" of the areas on the island, this will help you frame scenes more quickly later.
5) Determine some possible paths the adventure can take, so you can guide the new players via compels.
6) Draw up a short list of questions to assist the players in forming relations with one another. IE: One of you was reluctant to go on the trip, who talked you into it? One of you is excited to be shipwrecked who is it? etc.
When running the session:
1) Let the players pick the characters they want to play. Have them review the descriptive text so they will understand the mindset. Ask them the questions you prepared to help them get into the skins they are putting on.
2) Then take a moment to explain compels and fate points. Read your prepped text, and explain where the characters are, and how they got there.
-- At this point you are no doubt 1 hour in, or even 1.5 hours in depending on rule discussions --
3) Begin role playing.
4) Start with a Skill conflict, Creating a camp on island, foraging for food, water, having to cross a ravine to get to the waterfall, make containers, etc. This will be the chance for the players to use skills, and a great time to explain skills to them, not to mention they can get to know the island, discover a cave, and be compelled to pick up that weird idol thing.
-- One to two hours could be spent by now exploring and using skills --
5) Create a physical conflict, A seeming of the demon attacking them. This will be the chance for the players to use combat abilities, and a great time to explain stress to them. As well as moving the plot forward, the demon wants them off the island after all, and fear is a great motivator.
-- Lets face it combat can be slow One hour used up here --
6) Create a Mental/Social conflict, perhaps the demon has taken control of one of the kids. This is a chance for players with non-physical skills to shine, and once again move the plot foward. They have to learn via conflict, that the idol is controlling a player, and via social attacks the demon wants the party to do things they would not normally do. So the players have to destroy the idol or whatever.
-- end session and surprise time flew by you are at the 4-6 hour mark --
With all your prep work done, the game should flow smoothly, and at the end of a hopefully fun session the players will be excited about creating characters, and a location to continue in. Then you can take on the 4-6 hour session of city building, the 4-6 hour session of character building knowing the players are hooked and wanting to play more.
But this is just my 2 cents.
