Well, in the 3rd week of May some of the schools are not out yet so it won't be as bad as if you go during June or July. The weather is hot but not too bad. And IOA is more like a water park, so I would go there if you start feeling overheated. That will cool you off in a hurry!
You shouldn't have to worry about any of that anyway since you have 5 days to spend in the parks. That is more than enough time to do everything more than once. If it is too crowded to you or too hot then you can always do half a day and come back the next day. Five days is plenty and it is nice b/c you won't feel as rushed. Just remember you have to use all five days consecutively. In other words, if the first day you use your tickets is May 21, then you have to use the remaining days by May 25. After that, they are no longer good.
As far as Must Sees, everyone will tell you something different, but it depends on if you like thrill rides or mild rides or what.
For US, I would do MIB, Jimmy Neutron (the little ones will probably enjoy that), The Mummy, E.T., and Shrek if you enjoyed the movie and don't mind waiting in line. The kids will probably enjoy Fievel Land (I think that's what it is called.) We always skip that since we don't have kids yet. And don't forget some of the shows. They are pretty good too.
At IOA, I would do Spiderman (you have not experienced 3D until you do Spiderman!), Popeye, Ripsaw Falls, The Jurassic Park River Adventure...oh heck, I would do everything at IOA!!! And the kids will probably enjoy Suess Land which has The Cat in the Hat, etc.
Hope this helps some!