Only been to Mexico, but we absolutely love it. We've been 4 times with our kids (ages 6, 10 & 11) and are doing our 5th trip this April. There is a lot to do in Mexico, cenotes, zip lining, natural parks, snorkeling, etc. Or you can just stay at the resort as there is always something to do there as well. We stayed at Sandos Carocol 2 years ago and had a great time. Last year we stayed at Marina El Cid. I would highly recommend this property especially with the ages of your kids. They always had fun activities going on at the beach & the pool and there is a cliff jump and a waterslide in the pool. My kids spent hours on that every day. This year we're trying the Grand Palladium just for something different and because its supposed to have a great beach.
I think both DR and Mexico have their fans. For us, Mexico always just prices out so much cheaper so we have never tried DR. The Mexican people are wonderful, and we have never felt unsafe anywhere we have gone there. We have friends that would "never" go to Mexico, but I have no idea why! It is awesome.