The best you can do is submit a certified copy of the divorce decree that shows that you both had custody of the child prior to the divorce. The school records would show that they lived with you during the period post divorce. The divorce decree would also show that custody was not taken away from you and therefore you remained a custodial parent.
The requirement seems a little contrived. For example parents that are not divorced don't have custody orders. Similarly parents that have a spouse that abandoned the child don't have custody orders.
I would also ask the over 18 year old to submit an affidavit stating you were their custodial parent for the period after the divorce until they reached the age of majority.
You should include in your letter a certified copy of birth certificates showing you are the custodial parents.
A copy of the divorce showing that you are the parents and that custody was not taken away.
School records showing that the residence of the children was with you.
Affidavit from the children stating where they lived and that you were the primary custodial parent during that period.
If possible an affidavit from you ex indicating that you were the primary custodial parent of the children.
Your paragraph can reference all of these items.