My husband and I are first cousins. Two of our cousins are yeateem, and they are brother & sister. We have we were putting some amount of money in a separate account for them, keeping in mind that it will become easier for us to spend money at the time of their marraiges.Now few days back we decided to convert that money into gold, because the rate of gold is increasing day by day. We thought that by the time they grow up we will have enough money as well as Gold for them.Do we have to pay Zakat on their money and Gold?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If the two orphans you are asking about have received the money or your husband is testamentary guardian of them and takes this money on their behalf, after having separated it from his own money, in both cases the money belongs to the orphans and the al-Zakah is due on this money. This is the opinion of the majority of Muslim scholars; al-Malikiya, al-Shafiyah and al-Hanafiah are among them.
According to al-Ahnaf an orphan is obliged to pay only the zakah of crops, fruits and al-Zakah al-Fitr. However, the first opinion is preponderant one since al-Zakah is due on the money regardless of its owner.
On the other hand, if the orphans did not receive the money or your husband is not a testamentary guardian for them, then the money belongs to you. So, you have to pay its Zakah. Know the al-Zakah is only due after the money reaches the Nisaab (the minimum amount that is liable to pay al-Zakah) and after completing a lunar year in one's possession.
Allah knows best