The only beaches I've been to in St Maarten are Orient Beach (French side) and Great Bay Beach (Dutch side). I prefer Orient Beach, because I enjoy swimming there due to the robust surf and waves. There are plenty of bars and facilities there, but it should be noted that on far end of Orient Beach is Club Orient which is clothing optional, so with the teens, you might want to keep that in mind.
Great Bay Beach is easier to get to get to from the cruise pier via water taxis. There is plenty to do in Phillipsburg as well.
I cannot comment on the other beaches you have mentioned.
I have done Kim Sha beach. It was a lovely beach, not all lounge chairs are ocean front. If you are the second half off the bus you will most likely not get an ocean front chair. We spent most of our time there in the water or at the restaurant so the view from a lounge chair didn't matter. Loungers are included, but umbrellas were an extra $5 cash only. I thought this was a bit nickel and dime-ish but happily paid the additional $5. The beach was beautiful and the ocean was calm with not much sea weed about. There are local vendors that walk up and down the rows of chairs selling their goods and offered foot massages. They will move on with a kind 'no thank you' however, another one will be along right behind them. The restroom and lunch restaurant are a short walk away and lunch was delicious! Both my husband and I ordered the bang bang shrimp which is served as a sandwich on a large baguette with french fries. Lunch was served at a specific time and I felt this really interfered with our ocean time as lunch was scheduled right in the middle of our time there. I don't think it could be helped, but I would have preferred to arrive earlier in the day and had lunch towards the end of our time so we could have an uninterrupted stretch of beach time. Our excursion tickets stated that "nudity may be observed" in the fine print, and indeed it was! However the topless bathers were limited to just a few and only noticed this as we walked past them on our way to the restaurant. Overall this was beautiful beach excursion and I would rate it a solid "B". We also booked Secret Sands Beach on St. Thomas and this was a much more secluded beach with no vendors. Also calm, beautiful waters with little seaweed. No umbrellas offered, but natural shade can be found. Also- no lunch but beverages were included. If I were choosing one of these, Secret Sands on St. Thomas was my favorite of the two by far.
