I’ve played some RPGs with my son (7) and my daughter (10), e.g. Mausritter. Right now, I’m eyeing Monsterhearts for one-one-one play with my daughter. She is 10 years old and totally into various supernatural high school hijinks fiction (various shows on Disney+, Harry Potter-style novels etc.), i.e. mostly light-hearted fiction for teens approaching or entering puberty (i.e. some shy romance, but no sex, the threat of violence, but few if any actual casualties (or at least not onscreen), no serious issues like eating disorders etc.).
I have not played any PbtA games before, but have some limited (and mixed) experience with Dust Devils, The Pool, Primetime Adventures.
(a) Is Monsterhearts suited for one-on-one play, and if so, are there any major tips or pitfalls to be aware of?
(b) Is Monsterheart suitable for a ten-year old, or more specifivally for the type of content detailed above? Could it be adapted easily? Is the book’s artwork PG or PG-13 (or R or whatever)?
As Judd points out, teen sexuality is an explicit element of the mechanics of the game. If that’s not something that the player’s ready to foreground then skip Monsterhearts.
I would add that Monsterhearts particularly, via its Strings mechanic, loses a major driver of play if you only have one player. Monsterhearts belongs to a branch of PBTA that embraces players targeting each other with fictional provocations over a group of players responding to a GM’s provocations. I don’t think it would work at all for a 1-on-1 game.
My Monsterhearts campaign didn’t get far (3 or so sessions before I moved away) but it’s worth saying that nobody had sex in it.
If you’re looking for something Buffy-like, with a strong inter-party dynamic and simple rules, maybe try Monster of the Week? I haven’t played it personally but I’ve heard good things.
Monster of the Week’s a solid choice because you can tailor the weakness and the scariness of the monster to the needs of the table.
My niece didn’t like media about fighting monsters to kill them, she liked it when people made friends with the monsters at the end. I, at her age, by contrast, was a bloodthirsty little creep. In Monster of the Week you can have the weakness of the monster be “it is scared and needs someone to sing it a lullaby” if you want. Then discovering that and pulling it off is the adventure.