I'm sorry it took a while for me to finally see this post. That's a good idea to meet with Kid's son but not so soon. Do you plan to have Mitsui travel with Sera indefinitely or are you just following the flow of your character? It'd be a shame for them to part ways so soon after her meeting a capable partner. But it's your character and I'm fine with whatever you choose. 

For what's next, if we think about it they've already reached the outskirts of the city that is pretty much deserted and empty of any survivors. Sera would want to find some people she knew from the city, be it classmates or teachers. She doesn't know where her parents are (she only knows they're on a mission) and is having doubts on what they've done, so that's a internal dilemma she'll be dealing with.

And yeah, fighting with Karasuko would not be good right now. We could have them reach a small town to rest and it's suddenly attacked by Madness beasts with their guard down.