Y'all are going to have the BEST time!!
1) Use a
travel agent (or Costco as recommended above). You will get OBC with a travel agent (as mentioned above). This can be a lot of money to spend on board (or to pay for tips)! Note, that I do not recommend
AAA. My MIL used them for our first cruise, and the agent was not helpful (there was no OBC, no discussion of which side of the ship, deck, or dinner seating was preferable). I had her cancel and I booked with a travel agent I had worked with in the past.
2) Get three cabins. You can't double up families. Maybe a couple interior (for each of the families) and one verandah/oceanview (for the grandparents where everyone can hang out and little ones can sleep while moms/dads go out later) OR 3 oceanview together so that no one has to worry about the balcony on a verandah. Ask the parents of kids what they'd prefer-be sure mom and dad of each family chimes in. I had lots of opinions when we traveled with my MIL. She is gracious and listened to them all. I was grateful for that as a DIL.
3) Find out when the historical opening dates for your cruise have been (I feel like summer 2020 is coming up in the next month or two, but I can't recall). There's also a place on the boards (can't recall where, I stumble upon it every time) where you can see current and historical prices for cruises. Perhaps you can figure out which dates have been more reasonable in the past.
4) My youngest was 3 on our first cruise. We did a 4 night. It was perfect. I wasn't sure how the kids would do, and I wasn't sure how I'd feel spending that much together time with my in-laws/they with me. We loved it so much that we're doing a 7 night together soon.
Managing Costs: Also, the 4-night cruise was to Nassau, which none of us was jazzed about. We stayed on board to enjoy the ship. Not having an excursion(s) to pay for helped to keep us within our budget. I packed beach toys for CC so my kids would have them without me having to buy any at the last minute.
5) Talk about passports with your children. I feel strongly that everyone should have one, including the kids. You never know what could happen on the cruise or in port. And if you don't already travel internationally together, maybe this will spur the interest. The cost is not insignificant if everyone doesn't already have one, so it will need to be budgeted.
6) While on board, book a placeholder. You never know! 10% off a future cruise and OBCs can't be beat!
Happy Sailing!!!