My wife & I stayed at Sunhaus with our two daughters (7.5 and 3.5 years old) for the October half term and had an absolutely perfect stay from start to finish.
The booking process is really easy, with great comms from Irini throughout. And we were able to shift the date of our arrival and departure back by a day really simply when we needed to. They also sent us their recommendations of things to do and places to eat in Santorini, sorted transfers from the airport to the accomodation for us and Irini's husband Akis was there waiting for us when we arrived, to show us around and make sure we were settled in - going above and beyond the call of duty to bring the kids some beach toys and a storybook, plus bring us a pack of water from the local shop so that we had some to get us started (as you can't drink the tap water on Santorini). He also lives in the Sunhaus next door so was always there in case we needed anything.
The Sunhaus itself is really clean, beautifully designed, cosy and comfortable, with the essential utensils in the kitchen, plus oil, vinegar, salt, pepper, sugar and washing up liquid, which was a nice surprise. There's one double bedroom on the ground floor, which my wife & I slept in, whilst the kids slept on the mezzanine above the kitchen, which they found really fun. The only thing to note here is that we would not have liked our kids to have been much younger than they are now, as there is a bit of a drop from the mezzanine level that, whilst it has a single beam in the way, is best for kids to stay well away from just in case. Our two stuck to our instructions to take care, though, and all was well. From the mezzanine space you can also get out onto the roof terrace which overlooks the sea, which was an amazing place to sit and have lunch in the sun.
Then, as everyone has said, this place is amazingly located if you like the beach, being just a few metres walk away. It's also right next to Irini's sisters yummy gelato ice cream shop, which is an added bonus! The beach itself is fine black shingle/coarse sand, but comfortable on the feet - and there are some layers/areas of finer/soft sand. We found quite a big patch walking along the beach towards the cliffs one day, which, the kids loved making sandcastles from. The sea was nice and gentle for most of the week, being completely calm on a couple of days and only rougher on windier days. There are also quite a few fish to be spotted if you take your snorkle or goggles. And there's a great shower at the Sunhaus to get that salt and sand off when you come home.
We didn't hire a car whilst we were there, but actually found getting around on the buses pretty easy. From Perissa, all the buses go to Fira, where the bus station is a central point to get to all other major locations on the island. So, as well as wandering round Fira one day, we went to Akrotiri and Oia - all of which are well worth a trip. On our first day there, we also spotted a tourist train which went to another beach nearby at Vlychada, which you can't get to by bus, but sadly we discovered that the train had then stopped for the season, so didn't get to go. During our time there (21st - 28th Oct) lots of other places also began to close (inc the ice cream shop!) but there were still enough shops and restaurants for us to stay comfortably. And Akis said he would be there till mid November, with other bookings.
Overall, we would recommend staying here in a heartbeat and we know for sure that the kids would go back in a second if they could. So, if you're thinking of a trip to Santorini, and fancy a pad that's right on a gorgeous black beach (rather than on a cliffside somewhere), then I'd say you've found the right place.