Correctamundo.
My nephew had the effrontery to do the same (and he added a lie, telling us that a Catholic priest would be there as a witness.)
When it was clear that there was no Catholic priest present, and in FULL VIEW of my darling children, I simply got up and left the church.
I hope it made the same impression on my children as it did on your family. And trust me, it was probably as difficult for the parish priest to "read out" your Grandpa as it was for me to leave--(after all, I had shaved, showered, and shined the shoes...)
BTW, are you as, ahhh, nervous about the Iraq adventure as I am?