What's the best way for a beginner cyclist to carry children? My sister has two children, aged 4 and 2. The 4 year old is starting to get confident on his bike and my sister is finding she can't keep up with him on foot any more. She hasn't ridden since she was a kid and so isn't very confident on a bike. She will only be riding on sealed bike trails (not on road) for the foreseeable future.

I'm not convinced any of the "on bike seat" style of child carriers will work very well for her, because of the reduction in stability and her lack of practice/confidence.

I suggested a trailer but she was wary of the extra weight and the effort required going up hills. I still think this is the best option though.

The other alternatives, the bigger trikes with seating built in are possibly a bit too expensive.

So my question is what's the best compromise of safety, cost, stability, weight and convenience for transporting kids?
The best option is a trailer. The pull weight is fairly low, and the safety factor is far greater than on a child seat.

To summarize what is a very long discussion, the child in a seat can't protect themselves, should you crash, and there is no way for you to protect them by guaranteeing you don't crash.

A trailer has a lower center of gravity, a more stable structure, protection from sun and rain, and a more comfortable ride. It will be heavier to pull uphill, but on sealed bike paths, that is not typically a major issue.

See this question for more information.