Wouldn't bother with Eden Project tbh. Overcrowded, overpriced and too hot in the biomes for little ones.
Lost Gardens Of Heligan supposed to be really good.
Lappa Valley is good.
Heartlands is free and has a great playground plus interesting museum.
If you're NT members, Trerice and Lanhydrock are both fab.
Poldhu Cove on the Lizard is my favourite beach in Cornwall but a bit of a drive from where you are. Much nearer is Porthluney, not quite as pretty but great for kids, loads of rock pools to explore, has car park/ice cream/toilets.
Mevagissey is great for a wander round the harbour and has a dinky little aquarium and local museum (both free).
We were Bosinver regulars when ours were little. Lappa Valley was a firm favourite but its all outdoors so make sure you go on a dry day. They also used to like Newquay zoo. Not sure how much your little one would get out of Eden project at that age tbh.
Re beaches, depends what you mean by family friendly? Ours loved rock pooling at that age and the usual sandcastle building. If you ask Pat and Dave for recommendations for beaches they will be most forthcoming. And also the visitor books in each cottage will be very useful for ideas.
There is so much to do at Bosinver itself though that the week will fly by. We all used to absolutely love it, it's a fab place for little ones - perfect holiday! Enjoy!
We have a house in Cornwall. Have seen this place that looks good for a wet day http://www.kidzworldcornwall.co.uk/ our son is only 9 months so will try it next time we go. Just a warning but Newquay is not the nicest of places to go although a lot of the children's activities are based around there. We went to Dairyland near Newquay which we and our son loved. There is animals to feed, pony rides, soft play etc. Flambards is also great.