When the kids were younger we used to take them camping on the Welsh Coast at this campsite in Dyffryn Ardudwy, a few miles North of Barmouth.
The campsite is right next to a really nice beach and sand dunes that run for two miles. The nearest pub (with beer garden for the kids) can be reached on foot in 5 minutes via a traffic free route and Barmouth is only 10 minutes away on the train (which stops next to the pub).
From Barmouth you’ve got a really nice ride along the Mawddach Trail towards Dolgellau and there is a pub about 8 miles into the ride that provides a nice lunch stop. It’s completely flat, so ideal for a tag-a-long.
If you can get away for a day then Coed-y-Brenin is only 40 minutes away. The Pont Scethin route can be started from Dyffryn Ardudwy and Cadair Idris is only a short drive away.
If the weather is good then it’s a great place to go on holiday.