Thassos has some fantastic beaches. small enough island that you visit most of them in a day with a car. Kefalonia and Lefkas also have great beaches. Don’t be put off by Kavos, as Corfu is a huge island – the North is very picturesque, and some lovely resorts.
Went to Thassos back in ’79/80, on an early Club 18/30 holiday, before they became synonymous with drunken debauchery. Stayed in a spare room belonging to a local, pretty basic but very cheap back then. The island is lovely, the beaches near to Thassos town are ideal for kids, as the water is fairly shallow, and as its on the norther side of the island facing the mainland, very sheltered. It was possible to get organised boat trips around to beaches on the southern side, which face out into the Med, so it can be windier and the sea a bit rougher.
I got heatstroke after an afternoon there, it felt quite cool with the wind, and I underestimated the strength of the sun. Big mistake! I felt bloody awful, and got quite badly burned to boot! Fortunately my girlfriend, being half Chinese, and having naturally darker skin than my pale English bod, was ok and made trips into town to bring back soothing food and drink.
The point being that with kids, you really have to take great care, I failed to and really suffered as a result. Otherwise it’s a lovely place, and as far as I know, still relatively unspoiled.