If you drink a decent amount
then you got a steal......some of the drinks there can be a bit pricey.
However, it's a great hotel and I think you will be pleased.
As an aside, did you check the JW rates for the same dates and package type? Sometimes the JW is only a few dollars more per night; it's a much fancier hotel, and the restaurants there are better (the Italian restaurant there is phenomenal....Gustino's). The rooms are also much more elegant, and the hotel itself is less than 4 years old (although the CasaMagna did not feel worn at all).
You can use the pools at the JW, though I will admit, with kids, you may get some looks at the pool to the right as you are facing the ocean (nearer the Marriott) as that seemed to be 'adults only' when we were there.
As far as the King rooms, just FYI, I don't believe any of the ocean front rooms at the CasaMagna Marriott have kings.....seems, strange, I know. Maybe that has changed in the past year.
I haven't seen the real steal packages of $99 here in a long time.
Seeing how much is included in your package, you might be hard pressed to spend $300 while at the hotel, unless they allow you to use that towards spa treatments or the booze in the gift shop.
Some decent shopping in the JW if you are looking for silver jewelry. Their prices were very reasonable for a hotel shop. (That was a separate concession that would not let us charge to the room, so I assume Bonus Bucks would not count here either.) Of course, you should also take the bus that stops right outside the hotels; it's cheap, safe and direct. Don't hesitate because this is Mexico; the bus system runs well and will take you to lots of shops and bazaars.
We enjoyed a day trip by boat to Isla Mujeres. Got a 2 for 1 coupon in one of those books they hand out at the airport, and that came in very handy (under $50 rt for a family of 5). It was a fun excursion. You can ride around the island in a golf cart and the kids will have fun.
The water in Cancun looks rough but in my experience, it hasn't been. Of course, check your conditions daily, but coming from Long Island as I do where the beaches have a wicked riptide, the Cancun waves looked similar but actually were much more tame.
The Marriott staff was wonderful; attention to detail was the key here, and they did it well. A very caring staff, and I wrote to the GM after our trip to tell him just that.
I am sure you all will have a great time.