We should note that games are not games for children but are to be judged as the most serious things they do.

[De vray il faut noter, que les jeux des enfants ne sont pas jeux: & les faut juger en eux, comme leurs plus serieuses actions.]

Michel de Montaigne (1533-1592) French essayist

Essays, Book 1, ch. 23 “On Custom and Not Easily Changing an Accepted Law [De la Coustume et de Ne Changer Aisément une Loy Receüe]” (1572) (1.23) (1595) [tr. Screech (1987)]
    (Source)

(Source (French)). Alternate translations:

For truely it is to bee noted, that Childrens playes are not sports, and should be deemed as their most serious actions.
[tr. Florio (1603), ch. 22]

As it must be noted, that the plays of children are not in jest, but must be judged of as their most serious actions.
[tr. Cotton (1686)]

Indeed, it is to be noted, that the plays of children are not performed in play, but are to be judged in them as their most serious actions.
[tr. Cotton/Hazlitt (1877), ch. 22]

We must take note that the games of children are not games in their eyes; and we must regard these as their most serious actions.
[E.g. (1884)]

Indeed, it should be noted that the games of children are not games, and must be judged as their most serious acts.
[tr. Ives (1925), ch. 23]

Indeed it must be noted that children’s games are not games, and must be judged in children like their more serious actions.
[tr. Frame (1943), ch. 23]