Part of a very conservative European society, wherein most young women gain the whole of their "sex education" from their mothers on the night before their wedding day, Casper knows that now is certainly not the time to explain these sensitive details to his young and innocent daughter.
Casper feels the eyes of his daughter on him, waiting for the answer to this curious question, and soon he knows how to answer his treasured daughter. He gets up from his seat, pulls down his brief case off of the luggage rack. It's bulging with parts and pieces for watch repair.
Here's the actual words from the book, as told by Corrie.
Will you carry it off the train, Corrie?" he said.
I stood up and tugged at it. It was crammed with the watches and spare parts he had purchased that morning.
It's too heavy," I said.
Yes," he said, "and it would be a pretty poor father who would ask his little girl to carry such a load. It's the same way, Corrie, with knowledge. Some knowledge is too heavy for children. When you are older and stronger, you can bear it. For now you must trust me to carry it for you.”
In our present day when independence and stamina and personal strength are often given too much importance, it's hard for many of us to understand such a trusting relationship, in which we would accept such an answer to our curiosity. But God knows His children. He knows what we can handle and when. And He loves us, so He doesn't give us the answers to our questions when He knows we aren't strong enough to bear them. He asks us simply to trust Him.