I would rather not have children at our ceremony and reception. How do I word this in the invitation and not cause offense?

This is a tricky one, since you run the risk of causing people you'd otherwise want to be at your wedding to miss the day if they don't want to travel without their kids. Apart from putting the word around, via the future Mothers in law is normally best, one possible solution would be to state on the invitation that since you are not having children at the ceremony or reception (don't give reasons why, just make it clear that they are not invited) baby sitting facilities can be arranged for those guests who are unable to arrange their own. You could then hire a baby sitter or two to look after the children in a room or a coupel of rooms at your reception venue. It will mean an extra cost, but it shouldn't be too much and it could mean that you avoid putting too many noses out of joint.