Try here for starters - although by the end of the day I am sure many more will have posted with links that they have used in the past and been more than happy with.
http://www.orlandovillas.com/
My first suggestion would be to search for a villa of the size you want, for the dates you want and check the box that says 'send this enquiry to other x bed villa owners' You will then get replies back from only those that have availability on your dates - rather than having to trawl through them all only to find out they are not available.
The amount of villas available is huge - so you should never have to compromise on what you want. We always look for a villa that has at least the following:
South/south east facing pool - not overlooked and backing onto a lake or nature reserve.
Games room
PC with Wi-Fi access
Free international phone calls
The more double beds and ensuites the better - but obviously if you have smaller kids then that isnt an issue.
Make sure you include a list of your requirements on the enquiry form - it will cut down on the amount of replies from owners who cannot provide what you want.
Location wise - we had always stayed on H27 until our last trip, when we stayed at Cumbrian Lakes. We much preferred the latter as it seemed to be closer to the parks, 192, Old Town and a couple of Walmarts - although don't get me wrong, H27 isnt exactly a hundred miles away either.
It also depends on whether you want to stay on a development that has lots of ammenities on it - such as communal pool, tennis courts or club house. We have never considered these important but I know from reading these forums that it is an important part of some peoples holiday - it's all down to what you want.
I would suggest that you have many hours of searching in front of you, but it will be ultimately worth it if you manage to get the perfect villa for you and your family.