Cargobike. Hands down.
There are LOADS around here these days, i like to thnik we were leading the charge but its wonderful that on my sub 2km nursery run i always see a couple.
Weve had two, and Urban Arrow (Which we sold cause Mrs couldnt get on with it), and currently a Longtail Yuba El Mundo.
Both bought second hand from people whose kids had grown out of them, but they only grew out of them by being too cool to be seen sat in/on a cargo bike. Both aged 10/11 i think
try dragging a trailer with two 10 yr olds in it.
And (depending on your circumstances) they still have legs when your kids can ride their bikes. I was talking to a friend who has bought one recently, and his kids are just about old enough to ride into town on their own bikes, but he said its so much easier no having to heard them, and debate which way to go or get everyone organised. Throw them in an go.
They are pricey, compared to a trailer, but they are a different thing. A very very cheap to run second car.
if you live in a location that justifies it (further away than everything being in walking distance, but not so far you cant ride everywhere) they are a winner
Also, if you can at all afford it stump up for an electric one. It opens up the possibilities enormously.
Quicker, Easier, you can carry more stuff more comfortably, and you can cycle in presentable/normal clothes with two kids on board and not be a sweaty mess.
If we go out as a fam, Mrs rides the Yuba with the kids, and im struggling to keep up on my regular bike.