I'm certainly no CR expert but we have been to Montezuma (stayed in Cabuya), Sta. Teresa and up the coast to Samara as well as Playa Hermosa near the LIR airport. Best place of those in my opinion for a family beach vacation would still be Samara. Samara has more of an area to walk around with restaurants and some funky tourist shops and the beach is just one big long beach - great for long morning walks!! Nice easy swimming. I'm sure your kids will love it...assuming they love the beach and water. Montezuma has a "cool vibe" but I think the beach in Samara wins hands down.
Also good choices around Samara for other beach visits...easy to do with a 4x4 vehicle.
Yes, Sta. Teresa beach is absolutely gorgeous but if you're staying with your kids, you might regret it because the surf is quite big. I also got the feeling that there were many more bar celebrations geared towards the 20 something crowd...full moon parties, etc. Sounds like a lot of fun...but we are past that point in our lives and our family really enjoyed our time in Samara.