We've moved this topic to the Tamarindo forum, because it should be more likely to receive some replies here. We hope you find the information you're looking for.
I think the beaches in Manuel Antonio are safer swimming wise compared to Tamarindo. Having said that you need to be careful no matter where you swim, we have some tricky rip tides but the MA area tends to be pretty calm in general. MA gets my vote.
Keep transport times to and from the airport in mind, particularly with the kids. Tamarindo closer to Liberia (LIR), MA closer to San Jose/Alajuela (SJO).
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