How much are you planning on spending???
for 10 days I would go with a value resort - cheaper - I happen to love Pop Century - but people with small children seem love to All Star Movie a little more...
have you seen the pictures on this site
http://www.wdwinfo.com/photopost/showgallery.php?cat=504
now if you have the money - the Polyn is great - monrail access to MK - so you can just push your strollers on it.... but it is expensive!!!
Unless your 5 year is in school - I would go this fall - Sept, Oct, Nov, Dec.
I happen to love it the first weeks of Dec - everything is decorated and it is wonderful!!!
if he is in school that means during a school break. I would go with spring break (next year) unless it is Easter or the week before Easter.
then look at his calender - some schools have a fall break (great time to go) I even prefer Thanksgiving over summer.
summer in orlando can be hot, humdity and crowded... dealing the crowds is bad enough don't (unless you have no choice) add the heat.
more stuff for you to read and consider
http://www.wdwinfo.com/Just_for_kids/index.htm
http://www.wdwinfo.com/tips_for_touring/rookiefmm.htm
