Southwest Crete Beaches

kwoolever, we were in SW Crete last summer with kids similar ages. I cant comment on Gavdos bc we didnt go. We stayed in Loutro 3 nights and explored that coast. Marmara was one of our favorite days. We hiked there from Loutro which took about an hour (with a 10, 13 and 15 yo). There was one sketchy part where my 10yo almost fell over the cliff. Or it felt that way to me (my husband says she "stumbled in a precarious spot". All the kids thought I freaked out). Anyway, the hike was fun, the beach is small and rocky but if you get there early you will have a beach chair and there is a delicious restaurant next to the beach. Best of all, there are cliffs and coves for swimming. We were there in July and the water was still quite cold, but my 13yo son was elated cliff jumping. He made friends with all the kids his age and they all jumped all day as a posse. I read a book and ate. I took the ferry back to Loutro with the younger two kids and my husband hiked back with our eldest who likes to hike at every opportunity (the younger two dont ;) )
Loutro was a fun and relaxing place to hang out but very slow. Our first afternoon there we spent renting peddle boats and exploring the cover and coast that Loutro sits in. But one afternoon was enough. The evenings were great because all outside tourists leave and its like a camp, with bars along the water where everyone hangs out. We happened to be there during the World Cup so watched that with everyone every evening (we are futbol fans so yeah, great fun).
We didnt spend a lot of time in Chora Sfakion except to park and take the ferry to and from Loutro, but I really liked what I saw. I was surprised more people do not stay there (although can understand the novelty of Loutro). You cant lose, whichever you choose
We were also in Chania for 3 nights, and Heraklion for 1. Loved Knossos, and loved the archeological museum in Heraklion (we were only there for an afternoon and night before departing for Santorini).

FYI we overnighted in Paleohora and wouldn't spend time there again. It's not a particularly attractive city and the beaches were just OK. We loved Loutro on the other hand. Just enough development to give you a choice of places to stay and restaurants to choose from. If we happened to mention in conversation to Cretans that we were going to Loutro they were always very encouraging, talking about cherished childhood memories of vacations there. We got fogged in a Loutro for 1/2 a day (this was around the second week of June). The coast guard disallowed all craft, including ferries; it was very thick. It didn't occur to us that this could happen, although an older woman who was tending bar in Loutro was very certain of her ability to predict it. I suppose you just want to keep an eye on the forecast.

BTW Imbros Gorge is very close to Chora Sfakion if you're interested. Given the itinerary above I would spend the time on the water as you're planning instead of hiking the gorge.