What's the best way for a beginner cyclist to carry children?
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.