All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is an obligation on these children to take their mother's case to an Islamic court or to an authority that serves in its place, in order to investigate about her condition. It is probable that she had died before her husband, and therefore, it is her husband who inherits her, and the entire money is transferred to his heirs after his death.
It is also probable that he had died before her, and therefore, she deserves her share of his inheritance. In this case, her share should be preserved until her death is known, or, based on the judge's best judgement, the court issues a verdict that she is dead, because there is no hope that she is still alive.
Before such a ruling is issued, her heirs (or others) have no right to dispose of her money. Rather, an agent should be appointed to protect her money or to invest it, if there was no agent actually appointed before the person became missing.
The Kuwaiti Fiqh Encyclopedia reads:
“If the missing person does not have an agent to look after the collection of his money, protect it, and get all his rights, such as confirmed debts (not disputed), assets, and profits; if there is no appointed agent, then the judge must appoint an agent. The agent has no right to dispute (in court) about the rights of the missing person, except with the permission of the judge. He is also not allowed to dispute about the rights that are due on the missing person to others; this is the view of the Hanafi School and the Maaliki School agreed with them in general.” [End of quote]
Allah knows best.