First time cruisers might consider letting price dictate the specifics. My logic goes like this... fancy stuff would be wasted on you, lol! You'll come back with a few things you didn't like, and the next cruise, you'll know to avoid a cruise serving up those annoyances.
I've never used a travel agent. They might end up costing you more by not showing you an acceptable, cheaper option. "vacations to go" web site can give you the sailings and pricing. The price you are typically presented is per person, presuming double occupancy. Just put in the people and cabins and get to where you are about to buy and you will see how it works. You need to add $13 or so per adult per day and some lower number for kids for mandatory tips. This is a huge annoyance...if its mandatory, it's not a tip and should be in the price! You give them your credit card and the tips and anything else you buy goes on your "onboard account". This way spending on $12 rum drinks is painless...until you spend a decade paying off your credit card.
You will need to book one adult and one kid in each cabin, but when you get on board, you can put the two of them across the hall. They'll pay full price that way. It's unlikely you'd get adjoining outside rooms balcony/window. Until high school, we had our 2 kids in with us in a single room. We did plenty of cruises like that, no problem. But our two girls got along well, and we did everything together. Schedule the shower a bit is all. If your kids are used to day care, they might just love the kids club activities...you can drop them off and they can't "escape" unless you come to get them. All lines have kids "clubs", but Carnival and Princess have better ones and Disney, if you want to spend more. My kids felt imprisoned, but they never went to day care w/a sahm.
Port parking is $200 or so. You can find a hotel near the port that's got a shuttle and much cheaper parking. The off-site parking outfits suck at bringing you back to your car in a reasonable amount of time, but the hotel shuttle worked ok for us.
More ships are in the Caribbean in the winter. You can't count on the price dropping as sailing date approaches, especially when kids might be out of school. April/May you'll find a lot of low prices.
One last bit of advice: since all the staff get their "tip" income, whether they serve you well or not, your carrot and stick is the end of cruise survey. Good servers will remind you of their name again and again, lol! My advice is to read name tags and ask for pronunciation of their name. Repeat their name a few times when you see them and they will be your "friend" and you'll be able to give them the opportunity to earn their "tip".