The day after that, Christmas Eve, he went and bought four bicycles, one for each of his three kids, ages 16, 14 and 12, along with one for himself. He put them under the tree, a father’s promise that the future would be better. “The kids are why I’m alive,” he says. “God knows what lessons I’m teaching them. I don’t know, but I know I’m giving them hard shells.”
(Beyond the Breach 08/24/15 • ESPN the Magazine)
What does “hard shells” mean here?
From the context given my best guess would be that it probably means (for the parent) to bring them up (parenting) in such a manner that makes them very independent, can live life by themselves and have a high self esteem/not be affected by the opinions of others.