How is the zakat calculated on funds deposited in joint accounts of children between spouses, especially since the wife added her husband's money to hers, and it is no longer possible to determine each one's share?

If it is impossible to ascertain the amount of the deposit, one should investigate and act upon the predominant assumption, taking precautions in this regard. So, if one doubts whether the share is twenty thousand or thirty thousand, he should pay zakat (definition) on thirty thousand as a precaution. Ibn Uthaymeen mentioned that the estimation of zakat should be done by investigating as much as possible, and that the predominant assumption takes the place of certainty. It is always preferable to err on the side of caution in matters of religion.