Adulthood is a condition of being a witness to marriage

 

Praise be to Allaah.

In order for a marriage to
be valid, it is stipulated that it be witnessed by two Muslim witnesses of
good character, because the Prophet (peace and blessings of Allaah be
upon him) said: “There is no marriage except with a wali (guardian) and two
witnesses of good character. Narrated by al-Bayhaqi from the hadeeth of
‘Imraan and ‘Aa’ishah; classed as saheeh by al-Albaani in Saheeh
al-Jaami’ no. 7557. 

With regard to the witness,
it is stipulated that he be male, an adult and of sound mind. The testimony
of a child, woman or insane person is not valid. 

It says in Sharh
Muntaha’l-Iraadaat (2/648): The marriage contract cannot be done except
with the witness of two males who are adults, of sound mind, able to speak
and hear, Muslims – even if the wife is a dhimmi (Jew or Christian living
under Muslim rule) – and of good character, even if it is only outwardly.
End quote. 

It says in al-Mawsoo’ah
al-Fiqhiyyah (41/296): The Hanafis, Maalikis, Shaafa’is and Hanbalis are
of the view that the two witnesses to marriage must be mukallaf i.e., adults
of sound mind. The testimony of an insane person is not acceptable,
according to scholarly consensus. And a child cannot be a witness, because
the Prophet (peace and blessings of Allaah be upon him) said: “Bring
two witnesses from among your men,” because children are not among those who
can give testimony. End quote. 

The presence of witnesses
is one of the conditions of marriage being valid, whether the marriage
contract is done verbally or in writing, in the event that the husband or
wali is unable to speak. 

Based on that, it is not
valid for children to be witnesses to the marriage contract. 

And Allaah knows best.