Is it permissible to deposit a sum of money in a bank as a trust fund to be spent on the deceased's wife and infant son? Are the interest earnings from this deposit considered usury (riba)? And what should actually be done if it is unlawful?

If the funds are deposited in an Islamic bank and operate in accordance with Sharia (definition), then the acquired money is halal (definition) for the deceased's family. However, if they are in a usurious bank, then they are haram (definition) interest (riba), and they must be reclaimed and their interest disposed of by spending it on the welfare of Muslims. Then, it can be placed in an Islamic bank, or given to a trustworthy merchant for mudarabah (definition) (profit-sharing), or handed over to the deceased's family for their benefit.