S.D.
Actually, we found that anything in a sippy cup was let pass, bottles are probably the same. They also didn't bother us about things like diaper rash cream. You can take an empty water bottle and fill it after the security point to save money on water for you and your hubby. A few things to keep an 18-month-old busy:
a roll of tape
a small box of kleenex (just remember you have to clean up whatever they pull out)
my son was happiest with 2 cups from the drink cart and a spoon (go figure)
We thought a separate seat was a waste of money (the seats are over-priced as it is!). Both times we've flown with our older son, 3 months and 16 months, we had him on our laps. If the flight isn't full, switch to a row that has an extra seat.
Give the youngest something to drink on the way up and down to help with ears. Take suckers for the older.
Check car seats and strollers at the gate, don't take them on the plane. Or, rent them from the company if you are renting a car. Also, if you are staying in a hotel, ask if they have a portable crib or play pen for an extra charge, if not, there are local companies that rent anything you could possibly need for babies and kids. Just search on the internet for something close to you.