I am just going to cast another vote for the Faith and Life books together with the First Communion Catechism. We were overseas last year and while the kids were in Catholic school, they don’t do 1st Communion there until 4th grade. So I used the F&L book and workbook along with the First Communion Catechism at home. We got back to TX just a month before the parish kids made First Communion and the DRE was at first a little hesitant. But after the teacher “tested” him, she told me he was better prepared than most of the kids who had been in the program all year.
There is also a nice video you might want to look at: The Angel Force: The Mass. It is available from Catholic bookstores and from EWTN. It is the Mass as looked at from the viewpoint of a girl too young to receive the Eucharist yet. It is full of her longing and desire for Jesus as well as a nice reverent look at the Mass (Novus Ordo, by the way).
Good luck, these kids are sure lucky to have you as their grandma!!!