STARTING to come up? In Zone 5? When did you plant it, and what type of grass was it?

If the leaves begin to mat down, you'll need to deal with it. Do you have a blower that is capable of the job?

Reminds me of the scene from "Butch Cassidy and the Sundance Kid" where the kid doesn't want to jump because he'll drown as he can't swim and Butch tells him not to worry about drowning, the fall will kill him.

Yes it will suffocate the seedlings. Blow them off and compost them. Or mulch them back into the lawn.

Never leave accumulations of leaves on a lawn unless you have gone over them with a mower to cut them up into little pieces. They will then become mulch and be taken down to feed organic material into the soil.
Otherwise, they will mat down and smother your lawn--effectively killing it.

Andy, you seem surprised that her lawn is now STARTING to come up...its not unusual to seed at this time of year--its actually, THE BEST time since there is no competition from weeds to hinder the early grass.
What you have to do though is buy a grass seed that DOES sprout. Sometimes homeowners get exasperated when they overseed and nothing shows for weeks... if ever.
Besides wasted labor a lot of money often is for nothing too.

Steelskies, you can always look forward to repeating the seeding effort in spring WHEN THE SOIL HAS WARMED UP ABOVE 50úF---The soil---not the air temperature.