It's early days yet, but I'm starting to look at the options for moving Helen around by bicycle. The basic choices seem to be a front-mounted child seat, a rear-mounted child seat, a trailer, or some kind of front-load cargo bicycle or tricycle (bakfiets-style or modern variants on that).
There's a description of these options at BikeHub, and people can be seen using all of these around Oxford, but it's hard to get a feel for how they would work in practice. (This is complicated by not knowing where Camilla will be working next year, or which nursery Helen will be in.)
For the moment I'm ruling out a front-mounted child seat, as they seem to work best on bicycles with particular geometries. My sister Jenny swears by the (rear) child seat, and that seems like the option that would interfere least with normal cycling. A good quality child seat might be £120.
A trailer is I think my preferred option. Overlooking the superficial appearance of vulnerability and analysing the likely failure modes, a trailer seems the safest - it's low to the ground, will stay upright if I drop the bike, and the better ones have a roll cage and suspension harness. Consumer Reports has a brief analysis of this. It has the advantage that it could be attached to either my bike or Camilla's, so I could drop Helen off at nursery and Camilla could pick her up. And the fancier models can be converted to strollers, so we'd be able to cycle into town and then push Helen around. A high end child trailer might be £400, a cheaper one £170.
Camilla likes the idea of a cargo bicycle, with the child in a giant front "wheelbarrow", but that involves an entirely separate bicycle (or tricycle). I suspect this would take some getting used to with the steering, etc. and might be hard to park in normal bike parking. And a good quality cargo bike would be upwards of £1000.
What we really need is a chance to try all these out, so I'm hoping to organise some kind of "parent cycling" event, perhaps through Cyclox, where new parents can get advice from parents who are already using these various transport systems, and maybe try them out.
Update: Will Schreiber has put together an excellent overview of cargo bike options. I like the look of the Kangaroo tricycle, but it costs as much as a car!
Update 2: I ended up getting a Wee-Ride centre-mounted seat.
Rear-mounted seats worked well for both our kids, although the trailers were tempting. Did try a cargo bike and found it a little hard to maneuver, but that might just have been that particular model.
When they were too big for the seat, they graduated to a bike towed behind mine, so my old bike did the job through each stage.
Having used a trailer for two years in Basel, I can't recommend them highly enough. In addition to the advantages you've listed, you can pack in quite a bit of extra baggage as well as the child/children (we have twins). The kids are also protected from rain/snow fall. And you can leave the kids sitting in there while you do things nearby, which isn't possible with seats. Ours was a cheaper model that didn't convert to a stroller, but it only cost 250 Swiss Francs (£175).
As far as safety goes, I fell off my bike a couple of times at speed on slippery surfaces and the kids sat inside and watched in untroubled amusement.
Since you read German, this might be of interest:
A bewildering array of choice, all come with a hefty price tag. Tricycle is too cumbersome. You still have time to look at the alternatives, ask around. Helen can wait.
I'd vote trailer and my cycle group recommends them.
I know that the difference on stability and maneuverability between strapping a case of beer the top of my rack and breaking it up and putting in the panniers is quite large. And you are sticking your child's head at the end of a lever if you do fall over with a child seat.
Cargo bikes are designed to carry heavy bulky loads low down for stability. But the bikes are very heavy and hard to steer and not much use for anything else.
An electric cargo bike can replace a car. I use one for my 3 (7, 5 and 2) children to go to school and nursery and back. Not hard to steer at all.
I always worry about my bike tipping sideways when using a seat. Much less hassle than a trailer though.