I have four kids and have researched this multiple times, dreaming of a vacation when my kids were very little (and they still are--youngest is still just 4). Some of this info is from several year ago, so double check all of it.
For an all-inclusive, Paradisus Playa Del Carmen (and other Paradisus resorts) takes kids starting at 3. We almost went here several times but never made it.
https://www.melia.com/en/hotels/mexico/playa-del-carmen/paradisus-playa-del-carmen/services.htm
Beaches also takes little ones but it's so expensive and gets mediocre reviews.
We ended up actually going to Aulani in Hawaii back in 2016 b/c they take kids starting at 3. But since then, we've instead just done several Disney cruises, which we love. I really want to do an all-inclusive but can't convince DH.