You probably have already come to a solution to your bakfiets dilemma, but I thought I would put in my tuppence worth just in case you hadn't!
We have a bakfiets and love love love it. It is the short one with only one bench seat. I am 5'3" and dh is 5'11", and we both ride it easily. We live in central London, and our older girls are 7.5 and 6.5. I find it extremely stable, with one child, two children, 1 child plus groceries or 2 children plus groceries. It is extremely well engineered and has a very low centre of gravity so balance is never a problem. It also has a very low step through, so whenever you need to stop you put a foot down and that forms the 3rd leg of a tripod, if you see what I mean. It is very manoeuvrable.
Downsides - it is long, and it is heavy. If we lived in a hilly area it would not be workable. But any cycle that you have with that much weight on it (meaning the children, rather than the inherent weight of the bike) will be difficult with hills. The weight, however, makes it feel (and indeed, is,) very sturdy. It also means that it is very hard to knick! (Not to mention lack of street cred ).
It also has an internal lock on the back wheel, which is fab if you are just nipping into a shop, and a really good back rack, and a protected chain, so you don't have to faff around with cycle clips. It also has (optional extra) a special rain cover, which gives the kids / shopping total protection but also total visibility (and you look a little like you are driving a bat mobile!) which is something I would consider almost essential in this country - on an unprotected cycle they would have to wear rain trousers, etc and still have a somewhat unpleasant ride.
They do have strap harnesses on the bench seat.
I would be happy for anyone to come and give it a test ride if they like.