UK destination help with 9 year old

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Advice needed please.
UK destination for a week's holiday this summer, not too much more than an hour and a half / 100 miles from London.
Thinking Sussex.
Brighton, Bournemouth, or nearer Kent coast, Dymchurch.
OR Norfolk end we are thinking Great Yarmouth/Caister.

Any thoughts/ experiences recently greatly appreciated.

We stayed in deal, kent. Lovely. Stoney beach, lots of cafes and shops in the town. Towards the walmer end of town is a free paddling pool and also a mini golf place. Lots of days out in thecarea and other towns. Train station in town also

I live in Poole near Bournemouth, if you want any information. Norfolk is gorgeous, I know North Norfolk better, rather than Great Yarmouth area.

North Norfolk is better. Sandbanks - not Bournemouth. Chichester harbour. Aldeburgh or Woodbridge area in Suffolk.

Any nice areas/areas to avoid please?

Avoid great Yarmouth! Go to any town on North Norfolk coast. Go to Ssndbanks not Bournemouth.

Depends what you plan to do on your holiday. Beach? Museums? Outdoor activities?
Weymouth has a fantastic beach and there's an outdoor Sealife centre there.
It's not far from Swanage which has the steam train that goes to Corfe Castle (which is excellent if you're Enid Blyton fans). That's whole Weymouth, Portland, Swanage area is lovely.

My DCs loved Bournemouth when they were a similar age (and still now as teens). The beach is lovely, there’s lots to do in and around the town. Great Yarmouth is a bit depressing. North Norfolk is lovely.

Bournemouth is suffering a bit at the moment. Just been chatting to people who live nearby and shops have closed and too much rough sleeping. Beaches are great but other areas have more appeal.

Also if not bothered about beaches, the Cotswolds is just great for dc. Ours always loved our holidays there.