Something I do is attach the group to some other organization along the lines of The Harpers in the Forgotten Realms. If they want to strike out on their own they can, but for the most part their given small assignments by the organization. This helps tie together little adventures that may last a session or two while tying everything together. I also add in background stuff, rumors of bad guys, hints about a larger conspiracy and so on. The 11 year old likely won't care about that kind of stuff but the 14 year old will.
I normally just run campaigns from scratch, but there are also a ton of resources out there that people have eluded to. I got over my fear of being a bit silly long ago by just leaning into it and accepting that occasionally I'm going to look a bit silly. If your kids are having fun with it, that's all that matters. This is still a precious time when your kids actually want to play with you, embrace it!