Sorry this is a duplicate post as there are two threads on the same subject today.
Sounds ideal firsttimenanna.
Dependent on how far you are willing to travel might I suggest Weymouth.
Obviously a favourite of my family but it could tick all the boxes.
We always book with Dream Cottages and they have a good website and easy to speak to if you need advice, not pushy!
Weymouth has an excellent beach, a harbour, a marina, rockpools (a must for the little ones), good pubs and restaurants, cinema, shops, bowling alley, theatre RSPB Radiploe Lake, fishing trips, etc. etc.
Weymouth for the most part is on the flat except for one area called The Nothe but it is a lovely walk which takes you from the harbour , around Newtons Cove (where the rock pools are), up to the Fort and back down to the harbour. Ideal for rockpools, picnics, strolling and suits all ages. The thing is it is literally adjacent to the harbour and marina where it is all happening.
There is plenty of accommodation smack bang in the heart of Weymouth Centre where you literally will walk out of the door and you have it all on tap.
One suggestion if you are all dibbing in financially would be the 'Oast House' or properties in that vicinity, order a brochure and have a gander.
Have a wonderful time.