Recommendation for family hotel or resort.

Centreparcs - something for everyone and you can have a place with as much luxury as you want.

Centre parcs is nice, we took ours there a few years ago, but remember that every activity apart from the swimming pool cost extra. It’s not cheap and was very crowded in the activity area, and we went in term time. School hols will be very busy.
It’s our 50th this summer, I wanted to celebrate with the family too, but as we will no doubt end up paying for it all, dh wants to go on a cruise, just us two.

Crieff Hydro? Something for everyone there plus good food and friendly staff.

Crieff Hydro is excellent for all ages. The hotel itself could be a treat for the birthday members of the family and there are varying sizes of lodges to accommodate all your needs. Beautiful area and plenty of activities within the grounds for all ages. We were lucky with glorious weather last year.

Weymouth

Plenty of accommodation but I recommend renting a property on or around the harbour. Renting a property is cheaper, good for adults and children and price per head excellent value for larger parties.

Harbour, fishing trips, Rock pools, , good beach, shops, pubs and restaurants etc.

Living in Cornwall where else would I recommend.?. You have plenty of time to shop around and what better place than the internet.

Potters Leisure Resort in Lowestoft is highly recommended! We had a big family holiday there last year, our age range at the time was 6 months to 87 years. It’s all-inclusive and the food is marvellous, with lots of choices. Lots to do for all ages and free shows in the evenings. Enjoy!

I agree with Centreparks. I celebrated my 70th there last year. There were 3 dgc aged 6,3 and 2. We had 2 properties in the same road. A great success!

Alternative to Centre Parks is Darwin Forest Country Park.

Try renting a whole Youth Hostel. They are in all sorts of lovely places in the UK, very reasonably priced and fantastic for a big family. We did this a few years back for 24 people in one on Offa's Dyke so bikes and walking were well catered for.

The OP doesn't say whether she want s/c or catered. I really wouldn't advocate s/c with a large party who want time to enjoy themselves. Who will volunteer to cook and clean up? Not the birthday guests nor the anniversary couple I hope. Then there is the question of buying in food and who pays for what.
Who wants to get up and cook breakfast then wash up and make beds before going out...unless you go out to eat all your meals?

Royal Duchy Hotel in Falmouth. Family rooms beautiful food, often has good offers and extra things for children in the holidays. Heated pool and spa. Falmouth has lovely harbour, museum, nearby castle and is next door to St Ives with Tate gallery and lots to see as well. Lovely friendly staff who are retained for a long timebecause the hotel is not a chain and staff are treated well. I love it