Transporting 2 children by bike brings a new set of challenges compared to a single child. With Liam alone we used a Römer Jockey child seat, as shown below (not actually ours, I grabbed the pic from eBay).
We have 2 of these, so theoretically we can bring both kids anywhere we want to cycle. However, for visits to the Freibad, where a significant amount of towels, floating toys, sandwiches and sand toys are required, or indeed any situation where the adult:child ratio is less than 1:1, this is not applicable.
Thus once Rosalie was old enough, and the weather sunny enough (=end April) we splashed out on a Chariot Captain XL, shown with contents below.
The kids love it, and I'm impressed too. It's far easier to pull it than I had feared, the suspension soaks up the bumps for them pretty well. Uphill is still a struggle, but fortunately there's not too much of that on our regular routes anyway. The luggage space is pretty impressive, all "necessities" for the swimming pool can be brought along, including the (deflated) life-size dolphin.
The only catch is that Liam is so keen on cycling himself these days that he refuses to be carted anywhere less than 5 kilometres distance...
