Limewave,

Sounds FUN. Remember to keep it FUN. I get the feeling that you already know that but couldn't resist the reminder. If they think it's work...
When our kiddos were young we used to invite their friends to ride with us, the biggest issue was POS/department store bikes that put a damper on the outing as the bikes just couldn't perform and the kids were miserable. I don't have an answer to how to solve that one but certainly something to be aware of.
We would also do things like scavenger hunts on bikes, and of course, rides to the local ice cream parlor. Also follow the leader and all sorts of fun little games, especially when they start to get tired, took their minds off of pedaling and before you knew it they were laughing and smiling again.