Yes, as long as we know of the plus one before hand so we can plan seating arrangements and make sure we can feed everyone.
Of course, but there will be drinking around a bunch of adults. Therefore its your decision on if you want them to be around that type of crowd. After a certain time I think it'll be safer for children to not be present.
No, please bring your own. I know that's not the ideal answer but alcohol/bartenders are not cheap. If anything one of us could go pick it up from a local store and bring it to the venue for when you arrive if you can not find a way to purchase it yourself.
Anything! Just please be respectful to the bride and avoid wearing white! Suit and tie or jeans and boots are perfectly acceptable as long as you are comfortable.