Photographing children is a big part of many weddings, & it's something I've worked on over the years.
There are several approaches to doing this, I choose to interact as little as possible & photograph as an observer, it allows you access to those unguarded moments you may miss otherwise:
Observing people interacting with each other should almost always provide ample content for your photographs.
And you get the most natural & lasting photographs possible this way:
You can see more of my photographs here.