No secret website for rentals, unfortunately. There just aren't a huge number of rentals on the market. I'm happy to send you the names of the realtors I used for my recent search (one for the sound shore, one for the middle of Westchester and one for the Hudson). Just PM me.
I still wouldn't worry too much about schools at this point, even if you do end up staying longer. All of the towns you might consider have decent schools at the elementary level. Or if you stay and and you don't like the public school, you can decide to move to another town - not really a big deal for a five year old.
Keeping the commute to not much more than an hour means southern Westchester (below 287). As I said before, there's a ton to do with kids in the area, so as long as you have access to a car, where ever you land you can find many things to do with your boys. This website makes me a bit nauseous, but it does have some good ideas for things to do with kids in the area:
wee westchester
The towns along the sound with waterfront access are Pelham, New Rochelle, Larchmont, Mamaroneck and Rye (ok, Pelham only has a slice of waterfront, but it is considered on the Sound). The beaches on the sound are all pretty lame, in my opinion - I prefer actual waves, which requires going to Long Island itself. But for young kids the beaches are fine. And the parks/playgrounds are great. I've liked my experience in Larchmont so far - they have a newcomers club that's open to people from Larchmont, Mamaroneck and New Rochelle
Larchmont Newcomers Club: Home. Nice way to meet people in the community.
If you aren't committed to living near the sound, then pretty much any other town in Southern Westchester would be an option. They all have different flavors and characters which you can read about in the archives, but all have easy access to the city and provide very child-friendly places to live.
The only exceptions are Yonkers and Mount Vernon, which have some rougher areas you probably want to steer clear of. Even that said, while Yonkers is a mess as far as schools go, you could look at the Bronxville or Crestwood sections of Yonkers and find some cute houses in lovely neighborhoods. Mount Vernon also has one of the lower performing school districts, but the Fleetwood neighborhood could be an option (it's next to Bronxville).