Wait staff pools tips, so that when you tip the dining room staff, it end up going to everyone that serves. As the previous poster said, your dining room staff often does double duty in the Windjammer at different times, and if you're eating at the buffet for all meals, by tipping using autotip (you put it on your seapass account and get vouchers for the staff), the WJ people get money too.
Please remember that juice is only available during breakfast. So if your kids like it, bring an insulated type cup, pour some in and stick it in the fridge in your room for later on.
As for not eating with the family, that's certainly your choice. I'm not a big fan of late dining other than it gives you a bit more time when off the ship on excursions. But it's a bit late for my kids and they enjoy the evening activities in the clubs. If we did late seating, then by the time we're done, it could be close to 10 and then you're into late night club time (which means $5 per hour per kid). So either hit the buffet with the kids, feed them, put them in the clubs for a couple of hours and enjoy a nice meal with the family, or tell them sorry, we're eating casually with the kids and we'll see you the night of the wedding.
As for the specialty restaurants, I don't think they want kids under 13 or so in there. It's not for kids (plus it costs $20 for Portofino and I think $25 for Chops). So if you decide to go to one, put the kids in the club. FYI, there are about 3 nights on board when the kids can go to Johnny Rockets (burger/fries/onion ring place) with the kids club for free (otherwise there is a $3.95 cover charge). We did this with our boys the night we went to Portofino. Kids loved it, they were fed and went on to have fun, and we had a lovely meal.
