There are many reviews for the IB Grand under the 'Hotels' section of this forum. It's rated #1. Also, if you do a search in these forums, you'll find a lot of useful/helpful information.
It's been almost a year since I went to the Grand, and I will be returning again in six LONG months.
We loved the resort - everything about it.
The service is quite good, the staff is very friendly and accommodating. As far as cleanliness, I've never seen anything like it. I would have eaten off of the walkways. The rooms and all common areas were spotless at all times.
We went in to the spa area and it is gorgeous. We did not have spa services (they only do couples massages at the Grand, otherwise, you'll need to use the spa for the other IB resorts), but will probably try them next time.
The crowd is mixed. Some younger honeymooners. I'd say the average age is over the 20's, though. We saw couples, singles, groups traveling together.
Also keep in mind that your girls will be able to use the facilities at any of the other IB properties. If it gets a bit sleepy for them at the Grand, they can go to the disco or to search for other travelers who may be closer to their own age.
I would absolutely recommend the Grand. It all depends on what the three of you are looking to get out of this vacation. If you want more activity, I'm wondering if a resort closer to Playa del Carmen might be more of what you're looking for.
Don't know if this reply helped or added any confusion....
There's also http://www.iberostar-board.com - you'll find a lot of resort-specific information there.
Happy planning and enjoy. It sounds like a great trip you're planning.