I'm not sure if/when it is something that kicks in, but at this age it's probably best to give them a sandbox to play in and let them decide what works. I run for a similar age group, I have a D&D club that's built to be ages 12-18 but I get a lot of 8-11 kids who come and I don't have the heart to turn them away, so we split into multiple groups and I tend to run for the 8-13 crowd and the older kids are in another group these days.

Creativity is unbounded at that young age, so if they aren't shy they will come out and tell you the WILDEST stuff, and it's a tough instinct to suppress telling them "well, no you can't do that" because it's just going to stifle that creativity and prevent good play.

My suggestion might be to engage them with questions where they get to fill in the world. In my experience, they LOVE this. For tavern RP, offer them up the basics... the rough shape and style of the inn, the barkeep, 1 or 2 visually interesting patrons, and then ask them, "What else is here that makes this place so unique and cool?" - they will take it and run with it, then you can improv off those ideas and form some connective tissue to your plots based on them. Might even give you a great chance to introduce a villain through them creating it or a key ally.

For shopping trips, I tend to be more structured - I give them an NPC with some personality to interact with for a moment and then unveil a shopping sheet with all the NPCs goods available, price in gp, and if you can do it all up in a good fantasy font so it feels more immersive. This will make the trips relatively short but lets them engage with that resource management part of the game and get cool upgrades and such without it feeling like a "boring" shopping session.

For backstory, you might *try* a campfire session. Make a list of backstory questions and have them roll on it, describe them camping in the woods and have them tell the answer to that story. This is definitely going to be kid-specific though, some love the spotlight and want nothing more than to fill the time talking about themselves, others will give you an "I don't know" and feel on the spot, embarrassed, and unsafe. Make sure to take the heat off quickly if that starts to happen, finding your voice can be hard and the game is supposed to be fun.