Settlers of Catan is fantastic! My husband is not a big board game player (he prefers playing cards like Rook or Phase 10 or something), but he absolutely enjoys this one. Our kids do too (but they are older, 11 & 14).
If your kids are good at learning new games, and enjoy games like Monopoly, then they will pick this is without problems, even though the suggested age on the box is older than 6. There is no money involved, you simply roll the dice and collect supplies (based on the number rolled, and where you have settlements placed on the game board). There is a little bit of strategy involved in placing settlements, sort of like figuring out what is the best use of your money in Monopoly with buying houses/hotels on what properties. But nothing they can't do. Even if they don't do it well, they can do it and have fun.
The max # of people who can play (with the base game) is 4. But you can buy a supplemental set so that you can play with 6 people. That is what we did, because all 4 of us LOVE it, and so do my parents. So when they come, we can ALL play together!