And yet what harm can there be
in presenting the truth with a laugh, as teachers sometimes give
their children biscuits to coax them into learning their ABC?

[Quamquam ridentem dicere verum
quid vetat? ut pueris olim dant crustula blandi
doctores, elementa velint ut discere prima.]

Horace (65-8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Satires [Saturae, Sermones], Book 1, # 1, “Qui fit, Mæcenas,” l. 24ff (1.1.24-26) (35 BC) [tr. Rudd (2005 ed.)]
    (Source)

(Source (Latin)). Alternate translations:

Toyes may kepe and staye
Sumtimes the reeder very well, as those that teache in schooles,
With buttred bread, or featusse knacks will lewre the little fooles,
To learne a pace theyr A. B. C.
[tr. Drant (1567)]

Though to blurt out a truth has never been
(In way of merriment) esteem'd a sin.
The flattering Master thus his Boys presents
With Cakes, to make them learn their Rudiments.
[tr. A. B.; ed. Brome (1666)]

And mirth commends, and makes our Precepts take,
Thus Teachers bribe their Boys with Figs and Cake
To mind their books.
[tr. Creech (1684)]

Yet may not truth in laughing guise be drest?
As masters fondly sooth their boys to read
With cakes and sweetmeats.
[tr. Francis (1747)]

Albeit why may not truth in smiles be drest,
As gentle teachers lure the child to come
And learn his horn-book, with a sugar plum?
[tr. Howes (1845)]

Though what hinders one being merry, while telling the truth? as good-natured teachers at first give cakes to their boys, that they may be willing to learn their first rudiments.
[tr. Smart/Buckley (1853)]

Although what does prevent one telling truth in playful mood, as often tutors give their pupils cakes caressingly, to make them care to learn their ABC?
[tr. Millington (1870)]

Though, for me,
Why truth may not be gay, I cannot see:
Just as, we know, judicious teachers coax
With sugar-plum or cake their little folks
To learn their alphabet.
[tr. Conington (1874)]

What is to prevent one from telling truth as he laughs, even as teachers sometimes give cookies to children to coax them into learning their A B C?
[tr. Fairclough (Loeb) (1926)]

And yet -- there’s no law against telling the truth with a smile.
Smart teachers, for instance, give crunchy sweets to children
To make them learn their letters.
[tr. Palmer Bovie (1959)]

But tell me what law is violated if someone laughs
while speaking truth? You know how teachers sometimes give
their pupils little cakes, to help them learn their ABC’s.
[tr. Fuchs (1977)]

Though why can’t one tell the truth
With a smile? Teachers coax children to love
Learning by giving them cookies.
[tr. Raffel (1983)]

Although what's there to forbid one who is laughing,
from telling the truth? As loving teachers sometimes
hand out sweets to their pupils
so that they'll want to learn their ABC's.
[tr. Alexander (1999)]

Though what bars us
from telling truths with a laugh, the way teachers
sow cookies and reap memorized alphabets?
[tr. Matthews (2002)]

Though what stops one telling the truth
While smiling, as teachers often give children biscuits
To try and tempt them to learn their alphabet?
[tr. Kline (2015)]