No, nice shorts will NOT work at a country club for a rehersal dinner, and probaby not for the Bridal Luncheon either unless you're playing golf before hand, or the events are in the 19th Hole... no matter how nice the shorts are. It doesn't have to be an after 5 dress for the reception, but you don't want a sundress either. Stay away from cotton.
I'm going to suggest going to Kohls for a nice dress. They have great sales and their prices aren't too bad in the first place. Ross is also REALLY good, but you have to sift though the racks.
You're not going to like what I'm going to say about the babysitter: Plan to pay the babysitter several hundred dollars. I'm going to say $300 bare minimum. I don't know how much you pay her per hour, but you have to keep that in mind. If you normally pay $10/ hour, it's going to be different than if you pay her $5 or $15/ hr.
Most importantly, you CAN NOT pay her only for the hours she's babysitting because she's committing her entire weekend (2-3 days) to being with you. It's assumed that you're paying for her transportation and food. You wouldn't ask her to pay her own, would you? LOL. Even if you're paying for her food and "drinks" (??? I hope she's not drinking while she's taking care of your kids!) she's still not going to be able to go with you (unless she has a car with her,) and depending on how old she is, she maybe too young to hang out with the other teens whether she knows them or not.
Something else to consider is where the other parent that is supplying childcare is keeping the other children. Is it at the home of the bride and groom where the reception is being held? If not, maybe the bride and groom don't want kids in the house during the reception. I would not assume that it's okay to have the kids there.
If the child care provider IS keeping the other kids at the house, maybe you should reconsider letting your kids stay with the other provider since after all, you'll be in the house too.
Good luck with all of this! Don't forget to give us an update!