Those are great ages for tours! The Outback and Camel safari in Cabo would be perfect! Where else can they ride a camel in the dessert?
Plus it would be educational (yet FUN) and no snorkeling involved.
If you do want a great day of snorkeling, a trip to Las Caletas would be perfect. Very calm and pretty beach, a nature hike area, animals and GREAT food (and drinks) and a mini-zipline and a lot to do. You can even get a massage there at the Spa. The boat ride is fun also- we saw dolphins both times we went. It is really a nice day for all ages and lots of activities and if someone dives, they also offer a one tank dive.
Have fun! Both areas are VERY safe.