Hi P. - I'm thinking you may need to lighten up just a little bit. Just because they "see it in writing" wont increase their gratitude quotient. Saying grace is a great tradition but it's also much more than that. It's an acknowledgment that God is our provider in things big and small and that every good thing is a gift from above. Teach your children why you want to say grace.
Whether you say grace before food is served, during the meal or after the meal doesnt really matter. Decide the matter as a family. Maybe even have some fun with it and have each family member present and validate their "argument" and then vote as a family what your tradition will be.
Here's a prayer for you to surprise your family with next time they dig in before saying grace. "Dear God, please bless this food and all that is within me. Amen.
Truthfully, my cooking can be just a little sketchy sometimes. If it's a new recipe, we dont say grace before the meal, we say a prayer of protection! Ha!