知子莫若父 can be translated as “No one knows a son better than his father.”

This idiom is based on the idea that after spending many years together, family members know everything about each other. Better than anyone else, a father knows the qualities and shortcomings of his son.

If you are looking for something about “father and son,” this is probably the best selection.

While this is the original proverb (very old), others have been composed about various combinations of mothers, sons, daughters, and fathers. Let me know if you need a custom version.

子を持って知る親の恩 literally translates as: Only after you have a baby, you would appreciate your parents (feel the way they do, etc).

This is a bit like the “walk a mile in another man's shoes” saying. Basically, it's about you cannot fully understand the plight of others until you experience it yourself. It also shows appreciation for the plight of parents.

This Japanese proverb can also be translated a few more ways:

No man knows what he owes to his parents till he comes to have children of his own.

One knows not what one owes to one's parents till one comes to have children of one's own.

Only after you have a baby, you will appreciate your parents or feel the way they do.

Only after becoming a parent yourself do you realize how much you owe [how indebted you are] to your own parents.

Note: Because this selection contains some special Japanese Hiragana characters, it should be written by a Japanese calligrapher.