Seeing that you have family, the Royal Haciendas in Playa del Carmen is a great option - if you trade in there via II or RCI you will have to pay fees exchange fees + A/I fees, but you can always either rent from another owner or call the Royal Reservations booking center. RH has a great kids club (which may work for your 3 year old - the 6 mo old would be a little young...). They do T-shirt making, facepainting, pinatas party, etc. We started going there when my son was 5 and daughter was 3 and kids loved it. Unlike Vidanta mentioned above, RH is a short cab ride/drive from downtown Playa (10 minutes), so you can enjoy 5th avenue without a big hassle (great for little ones)
For renting from another owner, you can check on sites like TUG Marketplace, Redweek, OMCancun. You would want either a 1 bedroom Suite or a 2 bedroom Villa - when our kids were that young, the 1 bedroom worked perfectly - we had the master bedroom and the kids slept on pull-down trundles in the Living Room (or have a crib delivered). Based on the week, you should be able to get close to what the maintenance fee cost ($800 for 1 bedroom, $1200 for 2 bedroom) + $100-200 at the most (based on the renter and your willingness to bargain). Choosing Christmas, New Years, MLK or other higher demand weeks will be more, naturally. The big benefit of renting from an owner is then A/I is optional - you can do a none, a full week or # of days, so this can make a very affordable vacation, especially if you like to cook in your room.
If you don't want the hassle, you can call the Royal Reservation center (1-888-721-4469 - Stefano is great, if you ask for him). Many times based on the promotions they have going, you can get prices close to what renting costs. When going through the reservation center, since you aren't an owner you will have to purchase A/I, but they have a great deal when booking directly that kids are free, so you are only paying for the adults, so if you were planning on doing A/I anyway, it can be a wash. Also, if you go via making a reservation through a rep at the call center instead of online, they can sometimes guarantee you location (walkout, penthouse, phase, etc), so we always checked the various options.