I would mix the batter, refrigerate it overnight, and cook them in the morning. Will it be OK?
Absolutely! The batter always needs to rest, the longer the better. Just cover your bowl with cling wrap and stick it in the fridge overnight.
Actually, I think an overnight rest in the fridge is preferable to using the batter right after mixing it. I always do this. Crepe batter will benefit from at least a two-hour rest, but overnight is best.
Not only can you, but I highly recommend that you do. Your crepes will be lighter by far with an overnight rest.
Just curious...I never rest the batter. What will the difference be?