In Texas, purchase or possession is age 18.
Texas Penal Code - Section 46.06. Unlawful Transfer Of Certain Weapons - Texas Attorney Resources - Texas Laws
Penal code 46.06
(2) intentionally or knowingly sells, rents, leases,
or gives or offers to sell, rent, lease, or give to any child
younger than 18 years any firearm, club, or illegal knife;
So the OP is good to go. He is not selling it to the child, so he needs nothing in writnig(c) It is an affirmative defense to prosecution under
Subsection (a)(2) that the transfer was to a minor whose parent or
the person having legal custody of the minor had given written
permission for the sale or, if the transfer was other than a sale,
the parent or person having legal custody had given effective
consent.
Be sure to read it all;
So the OP is good to go. He is not selling it to the child, so he needs nothing in writnig