We've booked Trawsdir in Barmouth for the summer based on recommendations & great reviews.
The amenities are apparently spotless, there's a children's play area, the site not far from the beach & Barmouth is lovely to look around.
Fforest Fields, can't go wrong, lovely site, beautiful clean showers /toilets, lake to swim and boat in, ponds for the kids to go pond dipping and paddling. My favourite site in the UK.
I know a great site near Conway but it might be too far. Erw glas campsite. I went last year, toilets and showers are small but very clean and have heating.