Kids are tough to market to - haha, I think most companies want to advertise to kids (hoping for a long-term customer relationship).
I know for a lot of comic creators, Free Comic Book Day is a huge avenue to advertise to kids since most people that show up to these events are families looking for free stuff. I don't know what the situation is like for you area, but we have a local shop that runs a ton of family related events throughout the day (i.e. artists dropping by for free sketches) and it's a nice time to hand out some flyers or promote your work.
You can also contact local schools, try to get a zine printed. I donate a ton of comics to the local public schools in the area, they're ALWAYS looking for fun material for their kids to read. Local libraries are starting to feature zines as well.
Sure, these methods have relatively small reach, but the strength of your story should give word of mouth some legs to spread farther.
Instagram/Snapchat are great ways to advertise visual works, but again, you'll need a strong marketing plan to hope to grow big numbers - sometimes this is not as proactive as we'd like.
Looking forward to hearing from the creators here
Good thread topic!