I put these two beaches together because they are right next to each other - separated only by a small patch of picnic grounds. Use the same parking for both - a lot across the road (free). Be cautious when joining others to park on the street, we saw the police issuing parking tickets...
Poipu Beach is larger and offers a great place for even the youngest children to enjoy the water. Lava rocks offshore stop all wave action in places. In other places, the waves are mild. The water is shallow for a reasonable distance. One day a Monk Seal was making his appearance - as well as a Green Sea Turtle.
Snorkeling here is a favorite with many folks. Smart ones bring wetsuits... as in Feb at least, the water is cold. Wetsuits allow you to stay in MUCH longer. Don't have one? You can buy them in WalMart in Lihue... "shorties" are fine - those are the knee to elbow versions. When snorkeling, expect to see lots of different colored fish - and if you look carefully - eels too. They aren't afraid of people and they are plentiful. We were even lucky enough to spot an octopus - a favorite with all the children there that day. There isn't much coral on Kauai - it's too far north. There is some though.
Brennecke Beach is a great one for one main activity - boogie boarding! My boys brought the boogie boards in our condo and started as beginners by "trial and error." It wasn't long before some other tourists happening by were pointing them out thinking they were native sons since they were doing so well... (this can make a mama really proud!). I probably should have, but I didn't say a word - and somewhere, their pictures are in someone's album (besides ours).
One warning about Brennecke - there are some larger rocks underwater. 'Tis best to locate these BEFORE landing on them... There are submersed rocks at Poipu too, but it's easier to spot them when snorkeling!
A big advantage of these two beaches is the variety. We could let our boys boogie board, while those of us who preferred snorkeling, went to that side. In between was a great "green" area for sitting and watching the whales (need winter for this) and once we even saw Spinner Dolphins jumping. It's a great place to unwind, picnic, enjoy the water - or all of the above.