Can I play with just one kid?
Yes. Most one-kid tables give the kid two heroes so the scenes still have enough hands in them, and you play everybody they meet. Nothing in the kit needs rewriting for a small table, and the adventures run at their printed length.
What changes with one kid
On paper, almost nothing. Same adventure, same hero sheets, same hour. What changes is the room. With four kids they talk to each other, and with one they talk to you, so expect to do more of the talking and to field a lot more questions.
You also lose the gap where a kid thinks about their move while somebody else takes a turn. One kid gets every turn, which is wonderful for about forty minutes and then tiring, so put a break in earlier than you would with a full table.
Give them two heroes
Pick two of the seven and let your kid run both. The scenes were written for a party, and two heroes keeps the shape of them, so a fight has somebody to swing and somebody to help.
It also gives your kid somebody to argue with. Kids will happily hold both sides of a conversation in two voices, and a hard decision gets talked through out loud instead of stalling.
One hero works too, if two is more than your kid wants to run. Watch the fights, because a lone hero is the only thing in the room for a monster to go after.
If you would rather play one yourself
Take a hero, sit in the turn order, and be slightly useless on purpose. Ask the questions your kid has not thought of yet, then let them have the idea. A grown-up hero who solves the puzzle is the quickest way to make a kid stop trying.
You are still reading the boxed text and running the monsters, so give yourself the simplest sheet on the table.
If there are two kids
Two is a real table, and the only thing to watch is that neither of them is left waiting. Keep the scenes short and ask both of them what they are doing before you resolve anything.
Which adventure to start with
Adventure 01. Sixty minutes, one fight, one rescue, and it is the one written to be somebody’s first. Nothing in it is required for a subsequent adventure, so you can open at the bridge or the windmill instead if those are a better fit.