Ask Mr. Smarty Plants

You don't mention anything about the conditions in which the lawn needs to grow.  New construction generally means a lot that has been stripped of topsoil, there is little shade and what soil there is is compacted due to construction euqipment.  That means that most of the rain or irrigation water runs off instead of infiltrating the soil.  Needless to say, that is a very tough environment for any plant to live in.  In my experience, it takes about five years for a lawn to overcome these conditions and start to fill in.

If this pretty much describes your situation, you should be concentrating your efforts on improving your soil.  Aerate the soil and topdress with compost and natural fertilizer every fall and overseed while the soil is still warm.

There is a company in PA that specializes in Moss lawns but it is unlikely that your conditions are suitable, especially if you have children playing.  Check out Ecolawn, a mix of native grasses developed by one of our assosciates in Canada.  It could be just what you are looking for.  If they cannot ship to you, they will advise you where to obtain the seed in the US.  They will also be able to advise you whether to seed now or wait until spring.