Depends what you like and when you want to go ?
St Ives is my number one place, it is difficult to beat for sheer downright bloody gorgeousness, but heaving in the Summer hols and expensive to stay. Can't wait until we can visit late September.
Also in Cornwall, Looe is great if you have younger children, crabbing in either the big pool near the carpark which also has a park or off the harbour wall at either side. Great beach, very shallow shelf. Past the beach are some of the best rock pools I have ever looked in. West looe has Hannafore which tends to be a bit quieter, little rocky coves. Loads of different boat trips. Not far from Polperro which is worth a visit or Fowey which is lovely too. It does get busy in the Summer holidays but a lot are day trippers so it quitens down on an evening and depending on tide times we have hit the beach after tea and had a BBQ and there has been only a handful of people down there.
Barmouth has been mentioned, again busy during the day but gets much quieter in an evening, crabbing off the harbour, beach, hire mountain bikes and cycle across the bridge ( spectacular views ) , try your hand at paddle boarding in the harbour, if the children are older they do a trip up the estuary. Across the water you have Fairbourne with its cute little railway, you can get a ferry boat across and back if you don't want to walk round via the bridge. Loads of fabulous walks nearby too. Not too far from Portmerion, you can get a train and children go free and its a nice journey along the coast. Same for Aberdyfi in the other direction or get off in Tywyn and go on the Talyllyn railway there which is brilliant.
Whitby is lovely, as is Scarborough if you like a bit of old fashioned seaside fun, again it tends to quieten down in an evening at the front. Filey is smashing too as is Bridlington and Robin Hoods bay, all different in their way. If you stay in the general area they are all easy to get to and you can go inland and see the North Yorks moors, go to Pickering and get the steam train. Flamingo land isn't bad for a day out if you have under secondary age/early secondary age children.
Back to Wales, I have to say I do like Llandudno for a visit, Conwy is just around the corner which is lovely and you aren't that far from Betws y coed and the Zip world Fforest.
In Devon I really like Teignmouth and Exmouth,
Everyone I know who has been raves about Anglesey and the Isle of Wight for child friendly holidays.