It is at least a happy thought that your grandkids know and love and miss you. Any chance of skyping/the like or phone calls/face time-----or them coming to your place? It's OK to feel sad (I know I miss my grandson now that I don't live as close...but it is great when I see him)...they grow fast. (maybe they would like a couple of nights in a motel with you, give mom and dad a break? (or give mom and dad a date-overnight...? once a year or so, one with a pool --- or camping trip?... place to meet in the middle and do things as a family? …..exchange cards/photos...sorry I am babbling
Have a good cry and remember their smiles.