Where can I take my Mum for a chilled beach vacation for 5 days from Bangkok?

Hua Hin is the best option for a family friendly visit.

Beaches are okay and if you head a bit more south there are some beautiful places to visit.

Or Koh Larn near Pattaya, but there's not enough to do on that island for 5 days.

Brighton

Go to patong beach phuket 1hour 20minutes by plane

Go to Pattaya 90 minutes, then get a ferry to Koh Laan

I have been to Jomtiem , The hotels were reasonably priced .

Jomtien 90 minutes away. Easiest choice

Pattaya 90 minutes away

Hua Hin not that far you can go by train or take the bus

Take the VIP bus to Trat, then the songtaew to the ferry to Koh Chang island. It is the second largest island in Thailand. It is five hours from Bangkok. Make a hotel reservation in the White Sand Beach area. It is the perfect beach area to relax at close to Bangkok. In the evening there are fresh seafood restaurants set up on the beach, then fire twirling shows. Then there are the sunsets. It has one of the longest beaches just to walk on.

Make your way down to Cha-am about 3 hours out of Bangkok, great beachside community, and no rain here.