If you are worried about month to month cashflow (which, as far as I can tell, you don't need to be anyway) why do you want a 15 year loan? Just get a 30 year if that's your concern.
You are really asking whether you should invest a bunch of money in an illiquid instrument that returns 3.375%. Most people would say that's a dumb idea.
When you say "building a house" do you mean you are purchasing something under construction? Or that you are going to design/GC/build a house yourself? If the latter, having a kid at the same time is going to be quite a challenge. If the former, then you are just buying a house. If you have cold feet, ditch your $7500 and walk away.
-W