I know this is painful for you, but the first thing I hope you'll do is be thankful for the contacts you do have. Some of the saddest letters I receive are from parents and grandparents who never get to see their children or grandchildren, either because of distance or the terms of a divorce.
What can you do? First, make the most of the contacts you do have. Don't try to buy your grandchildren's affection by spending lavishly on them or entertaining them every moment they're with you; that probably would hurt your relationship with their mother. But let them know you love them, and make your time with them something they'll always remember.
In addition, don't overlook special occasions -- birthdays, holidays, and so forth. If you can, give them your e-mail address so they can communicate with you. I know several grandparents who've gone out of their way to learn e-mail or other types of social media so they can keep in touch with their grandchildren. Be sensitive, however, both to their desire for such contacts and the feelings of their mother. You don't want her to think you're trying to interfere.
Most of all, pray for your grandchildren -- and for their parents. You can't change the past, but God can change the future. Pray also for yourself, that they will see Christ's love in you. The Bible says, "Set an example... in speech, in life, in love, in faith and in purity" (1 Timothy 4:12).