I was in your situation this summer. Have you ever traveled internationally before and/or taken an overnight flight? If not, you don't know how you will do, or how your kids will do. This summer, 3 of the 4 of us (myself, DH, and 11 yo) did not sleep a wink to Barcelona. My 8 yo got about 3 hours. When we landed in Barcelona, it took about 90 minutes to get through customs. We took taxis to our hotel - it was very easy finding the taxi line. I would get two taxis - one with you and the kids, one with spouse and the 80-yos. Have slips of paper pre-printed with the hotel addresses on them to hand to the driver in case s/he does not speak English. To me, finding the taxi stand is much easier than searching for one specific person who may or may not be holding up your name on a sign. The taxis are plentiful and well-regulated and take credit cards.
Regarding things to do, you may (MAY) have the energy to walk around a bit that afternoon but I wouldn't make any firm plans given the ages of 4 of your traveling companions. The morning of the cruise, I recommend making reservations to Sagrada Familia (book 3 months out, as soon as the dates open - it sells out quickly) for their first tour (9 am). After that, taxi back to your hotel, pick up your luggage, and head to the ship. Given the size of your travel party 2 taxis will be much easier and about the same cost (or cheaper) than using the bus or metro systems. Plus navigating the subway with two kids and two elderly companions could be difficult.
What part of town are you staying in? We can provide suggestions for things to do that first evening if we know what landmarks you are staying near.
