1) Is it a good place with lots of activities for toddler?
I wouldn't say LOTS of activities but they will still enjoy it.
2) is it safe, free of cartels?
There's no state or region free of cartels in Mexico. Meaning they do business in every state. In some regions more than others. But the average visitor would never encounter them in Los Cabos. So it's not an issue at this time.
3) what is the best time to visit? Winter, less humidity. Or any other time that's not hurricane season.