We did this twice (one week each time) with 6 adults (us, PIL and SILs family) and 4 kids ranging from toddler to teenager. Great every time.
Make sure you have at least two cars to allow for separate plans and excursions. We even managed to 'park' the kids with the grandparents one day and 'get some culture' under our belt. Agree beforehand about how you will organise meals/ food, shopping and MONEY for this.
We go through this website villas and it is brilliant - you get far more for your money than going through a travel-agent type one. And you can search for exactly what you want. We go every year with my Mum and Dad. Couldn't recommend it more.
We do this all the time. Key tip is to get something big enough for all of you so you dont bump into each other all the time and be clear upfront on what the payment mechanisms are for both accommodation and also groceries.
What works for us is to head off and do our own thing during the day and meet up in the evening for dinner and games. That is the only meal we have together - everyone gets their own breakfast and lunch is generally outside.
What also helps is to have some sort of general idea on what dinner plans are - eating out (need to book?) or cooking and if so who is in charge.