I'm not going to say I'm a great campaign writer, but I'll hit you with my thoughts on the subject.
The XMFL-verse is one I'm less familiar with, mainly in that Monsters are actually accepted by society (to the point where they're used in sporting events). As for plot hooks, I believe the campaign jumpstart comes with a few.
I mean, if you want to run a campaign about kids who use their monsters in gladiatorial sporting events, draw on all the main hooks from other media and real life. There are all kinds of stories about sports and the players thereof, ranging from the underdog to champ who becomes jaded and leaves his friends, etc.
Regardless of the players' standing in the XMFL, simply being a participant is probably going to give them some level of celebrity (more if they're a favorite). Something about trying to maintain a normal life of friends, school and hanging out when you've got fans, advertisers, managers, etc.
Also, you're using your best friend as a piece in a game by having him fight and wound others constantly. There can be some drama between a kid and their monster.
There are all kinds of tropes you can draw on from this setting. Additionally, see what other stories your players are making for themselves.
As for consequences with monsters and stuff, this setting is kind of a double-edged sword. Everyone knows everything about monsters and are pretty cool with them as long as you don't start some shit. However, everyone knows everything about monsters and their kids (which is to say that it's pretty hard to hide monsters in this universe).
The MIB is probably more hesitant to act against monster kids, but when they do, it will be fast and brutal: They know everything about the kid, their monster, their friends, family, etc. Moreso if they're actually participating in the XMFL. If they're not, the setting puts forth the idea that monsters are typically used as tools, provided they have anything that can help people. Monsters (and by extension their kids) are now part of the labor force. How they're treated there is obviously up to you, but it's a big thing in this setting.
Additionally, if your monster is used for any criminal activity, the police are obviously going to be aware of the supernatural and come after the kid or their monster if it's apparent that they've committed this crime, and since monster kids are normally celebrities, the manhunt will be pretty quick (or outsourced to an aforementioned governmental division). Additionally, regular criminals might try to make crimes appear monstrous by nature, provided there's a monster in town that people would be uneasy about.
The XMFL is a good place tone-wise as it's more on the "Saturday Morning Cartoon" side of MaOCT, but still has the potential for drama. Just be aware that if a monster fights a regular person, you are going to have a drastic shift in tone very quickly. Those kinds of fights will never end without a huge mess (in both senses).
Anyway, that's my $2. MaOCT is pretty easy if you let players make their own stories. Find out what they want to do in the game and create NPCs that would fit in those storylines, in addition to any you have yourself. Like a superhero game or an epic-level D&D game, don't try to plan things out too rigidly as Monsters have a tendency to eat plots and mend plotholes.