The easiest way to approach this is to include on your invitations if this is a kid friendly event. You can include a note saying that as much as you love their children, due to budget or space issues, you cannot have children at the wedding.
Ultimately remember that this is your wedding and most parents will understand and probably be thrilled to have a date night out!
If you have a flower girl and ring bearer but still want the reception to be adults only, you can always ask their parents to take them home after the ceremony. Because how cute are these kids?

Flower Girl Dresses by Somsi Couture
If you have a question you'd like to Ask Abby, email Abby@sandiegostyleweddings.com.