Dear Maureen,

You didn't say how old your grandchildren are or if their parents were also present.  It seems though that if they are younger it would be easy enough to just have them stay in the pews at communion time.

But if they are older teens or adults and seem to be set on commiting sacrilege against the Holy Sacrament only so that they may "feel welcomed", then your task might become a little more difficult.  I suppose this could be argued, but in my OPINION, once you've made your position known real well just ONCE, you're okay.  The reason why I believe this is because it's actually the responsibility of the priest to safeguard the Eucharist, and ultimately the bishop.

I see no problem at all with pastors putting notices in the bulletin or signs at the entrances which charitably remind visitors who may and may not receive communion.

As for your question how to advise them, that all depends.  If they aren't practicing catholics at all, I would mention that receiving Communion affirms an individual's union with the entire Church, including all of the Church's teachings.  So if they have left because of some doctrinal issue, then it is they who have separated themselves from unity with the Church, and therefore Holy Communion.

On the other hand, if they are "cultural" catholics who don't attend Mass because of laziness or whatever reason, then I would take the approach of reminding them that Communion is to be received only by those who are in the state of grace.  Intentionaly missing Sunday Mass is considered grave matter, and so when the other two conditions are met we have mortal sin which removes us from the state of grace.  The other two conditions are full knowledge and full consent.

Finally, a note about the "feeling welcome" issue:  I know that many people are turned off by the idea of not being able to receive communion and find it terribly unwelcoming.  I would tell them they they still are of course welcome to otherwise attend Holy Mass, and that they are welcome someday soon to fully accept the teachings of the church that Christ founded, and so they are welcome to become or return as a full Catholic in full communion.

Thanks for writing,

Mr. Slavek