There was a time when you'd only see 2 parents, one mum and one dad, in any children's book or TV program. But now there are some nice examples of content that show families of all shapes and sizes.
Kit 'n' Kate, which kids can watch on Curious World, shows two kittens and their mum. Even though their mum being a single mum isn't a heavy feature of the series, subtly including different family dynamics in stories give kids a more realistic impression of what 'family' means.
The Family Book by Todd Parr is a book that is written to especially celebrate all different types of families. It's light hearted and reassuring and is a great way to introduce kids to lots of different ways that families are made up.
If some of your friends have a different family set up to your own, spending the day with them can help to open your child's eyes to how distinct every family is.
If your child comes home from preschool or school and has a few questions about your family, (or their friend's family) there's no need to over-explain. Just be straight with them and tell them the family situation. They'll be looking to see how you react, so treating these conversations like they're no big deal will show them that there's no problem with having a different family to their classmates.