Woooo a new face
1) Hmm, well theres many workarounds for playing with 1 or 2 PC's, and a lot of drawbacks to 6+ PC's. Generally for most PnP games, 4 or so seems to be the ideal number. I started with just me and a friend playing together, where he just played 2 characters (bully and dreamer). If I get another player (which I'm anticipating will be soon), then they'd probably create another character or 2 to fill in what's lacking.
2) Ummm that could vary tremendously. Perhaps you modify the Three "Little" Pigs adventure to be more fitting for a 3rd grader, or you could even start them at 5th or so grade, and pull off a one shot where the kids overhear the guards and royal officials of a small city talking about the king's scouts going missing in the World's Edge Mountains while you're creeping through the sewers. They decide to check it out and find various traps along a well traveled path only to find a brick house and 3 fat pigs waiting on the other side. The campaign is over when they kill the mighty wolf and free the pigs, and they share their secrets on the way home. OR, as the book suggests, you decide to make the kids' objective to defeat the Rotten King and end his reign. This could take a similarly long time as your Dark Heresy campaign as there are SEVERAL adventures that could be used from the book, perhaps every 2 adventures they graduate. The time is easily at the narrator's discretion, trust me when I say this - brainstorm brainstorm brainstorm! I made the most amature and embaressing mistake with my first GM attempt and only planned one house, and only thought out a linear chain of events. The just walked out the front door and avoided everything I thought up and left me dumbfounded. Running an 18 month campaign, I'm sure you know all those, but think on the broad and general scale and get more specific where you find necessary.
3) From my experience: A lot and a little, respectively.
- My player's keepsakes are the baseball bat and the magic marker. Although I have modified the rules on them a bit for roleplaying's sake - the bat doesn't require imagination to use for a ranged attack, the bully was the star slugger of his little league team and knew how to use it...why does he need imagination (he can expend muscle to increase damage though) and the dreamer's wild imagination makes it almost natural to be able to draw simple doors though foot thick walls without imagination (at the most), she can expend imagination to increase its capabilities through thicker objects and walls eventually creating portals (full imagination required....i know it breaks the rules) - keepsakes are in constant use in my party, from impressing villagers with even just the simple writing ability of the marker, let alone the door factor, threating corrupted gnomes to comply to the bully's demands with the aid of his bat, there are tons of uses.
- For magic, I've really only used it twice, the players haven't gotten to touch it. The dreamer is headed towards the magic path, but not their quite yet. Basically they have a wishing rock that they promised to return to its quarry and in return he keeps up the "Fortune" (guardian 1st circle spell) spell on the kids as long as they don't deviate from the path. If they do deviate, it just gets frustrated and removes the spell, waiting to recast it once they're on track again. The second time I used a spell was a sort if combo. The bully got "cursed" with a Charm spell (Enchanter first 1st-circle), but after using it's effects with a cool test, the spell then wore off and switched to Blemish (2nd-circle Witch spell). This was quite humorous because this was implemented while trying to swoon the Queen of Tin & Spices. Just as he thought he succeeded his face began to melt, break out, and contort cause the Queen to freak and he failed the test. But there are several chances later on in the game for players to be able to break out the wand and start spewing "Abra Kadabra".
Btw, I checked out your posts in the Dark Heresy forums...and my god do you provide a lot of input. If we get the same kind of treatment I can't wait.