I've been running errands and transporting my 4 year old son around town in a bike trailer since March or so (after requesting MMM community help to choose a bike trailer!). It's been fantastic so far, and we both love it, but I think we're outgrowing this solution. My son is very tall, and he no longer fits in the trailer without the top pressing against his head. It's fabric, but he still finds it annoying. He also has started to express dissatisfaction with being far behind me.
He can ride his own bike, but he doesn't have the endurance to do so for normal transportation and errands. I also don't trust him yet to interact appropriately and safely with other bikes and cars and people.
What's the best way to transport a child who is a bit too big for a trailer, but not yet mature enough/strong enough to ride solo? Are there modifications/add-ons I can get for my bike? Do I need to purchase a new bike that is designed to haul kids on a back seat? I only have one child to worry about, so the most I would ever need to haul is one child + some groceries.