Carrying your little one(s) can be so much easier by just popping them into a seat on your bike. Faster and easier than hooking up a trailer. And, usually easier than pulling something long and low behind you.
They're right up there with you, front or back, and are always happy to tell you where to go (which never really goes away, it seems).
Until they are ready to roll on their own, there are some great options for keeping them close.
Yepp/Thule Maxi or Mini, for rear or front, Blackburn CoPilot series, as well as the Evo Deluxe Baby Seat. Most child seats hold up to either 38 or 50lbs (depending on if front or rear), and you need to have a rack that will hold that weight, as well.