Put your child in front of you
Sure you can put your child on a seat behind you on a bike or in a child bike trailer but putting your toddler right in front of you where you can keep an eye on the child and interact is better.
One of the most popular bike accessory than allows you to do so is the iBert safe-T-Seat. Placed safely between the rider's arm, right behind the handlebar, the iBert has a mock padded steering wheel the child can play with and pretend to steer as you take her for a ride. It's a lot safer than bike trailers and you don't have to worry about rocks or dirt getting in your child's face.
Installing the iBert on your bike on takes a few minutes and all the tools you need come in the box. It fits just about any bike though for some bikes, like the cruiser bike, you may need to raise the steering tube a little bit. Oonce installed, the seat can be put on and taken off the bike in seconds. The straps and cross bar keep you child safely in place.
Having your child on a seat in front of you as opposed to on a trailer behind you seems like a better option in terms of safety and practicality.