My belief is that many times in the case of divorced parents..the grandparents need to step up to the plate...just as you are doing.
I do hope they are involved in a loving way.
Their version of caring for the kids is buying them stuff ..... they don't seem to know the meaning of unselfish love. The kids will have the best of everything that money can buy as long as their paternal grandparents are in the picture ...... but no real affection .....
.... an example ...... when grandaughter was about a year old and they were staying with us, the kids spent a week with their other grandparents. When they came back, grandaughter's bangs had been trimmed very unevenly. The Jerk enjoyed telling us that his mother said she had to take three drinks to get up the nerve to cut the child's hair!!!!!! YIKES! Three drinks and then puts scissors up to that baby's face! They all think it's hilarious. Just like the tale of The Jerk's niece getting kicked out of day care for being too unmanageable ..... *sigh*