Even within England, if you live in the NE, then Cornwall is a long way. If you live in Kent, I probably wouldn't want to drive to West Wales.
Travel aside, I love Pembrokeshire. Loads of lovely, quiet beaches there. Norfolk has lovely white / soft sandy beaches with sand dunes. Dorset and Devon have some nice beaches too, and probably more bigger seaside towns. Cornwall is, of course lovely for beaches - lots of little coves etc., but the roads are narrow and the place is very usy at that time of year.
If you are near the south the Isle of Wight is great for young kids - the ferry crossing over is exciting for them, there are loads of fab beaches and plenty of other attractions within a small area, depending on what you like - animal parks, a castle, steam trains, theme parks etc
Lake District IMO when weather is good, loads of walks suitable especially if you have a good off road buggy. What accommodation are you after? The Langdale Estate has great facilities including pool and playroom, with self catering lodges on site
Isle of Mull is good. My boys enjoyed the beaches, boat trips, seal watching, the colourful houses in Tobermory (aka Balamory). I was going to say the miniature railway but it seems that's shut down now
Southerness Holiday Village has a toddlers' pool, indoor soft play and a kids club for under 5s. There's a kids club for older children too, a flume in the indoor swimming pool and adventure play area. The pool has inflatable assault course sessions.
Craig Tara in southwest Scotland has loads for your oldest DC, including an amazing swimming pool complex and rock climbing. There are forest-school type sessions for a small additional cost that didn't seem to have a lower age limit so I reckon your toddler could join in if you accompany. They have go karts and a scooter school for children aged 2-6 and lots of art activities for all ages.
Aviemore. One of the big resorts, e.g. Hilton coylumbridge or Macdonald. They have soft play for the little one. Lovely outdoor playgrounds. Then also more exciting outdoor activities for the eldest. Landmark is fab, the strathspey railway steam train, the beach at Loch Morlich, cable cars up Cairngorm, highland wildlife park are only a some of the activities that come to mind.
I'd second the Isle of Wight for that age group, something different to do everyday. Google Classic Cottages / Wight Locations ....July is filling fast though, I know as I'm a cottage owner there. Good luck!