Listen. Never condemn their choices, but ask them what they hope to achieve by their actions. If it turns out badly, comfort them and gently steer them the next time - children need you to affirm their choices. Obviously don't let them do something dangerous to their health/safety/general rules. Never make them feel that you are more clever that they are; simply point out that you have been around longer, so have had to make more decisions and, with luck, you've learned from them.Try to explain the rules of an ordered society, point out that everyone is different and when people get together, sometimes the little voice is the correct one, not the person who shouts the loudest/has most 'friends'/wears the weirdest clothes/etc. Ask them to always tell them if they are being bullied, in school or out - they might not have realised that you, as well as their parents, can step in (and that you might have more time). Show them how to explore any opportunity, whether in the park, swimming pool or elsewhere. Tell them that they can become anything they want, but free choice requires more effort, so it's no good saying 'I can't do maths' for example. All problems have different solutions so if they can't 'do' fractions, perhaps working out ratios makes more sense to them.
Above all,smile and enjoy yourselves,your grandchildren will grow up even faster that your children did! 