Are children considered witnesses in a verbal marriage contract?

For a marriage to be valid, there must be two just Muslim witnesses, who are adult, sane, and male; this is based on the Prophet's (peace and blessings be upon him) saying: "There is no marriage except with a guardian and two just witnesses." Also, the testimony of a child or an insane person is not accepted. Witnessing is a condition for the contract, whether it is verbal or written.