1. If it were to be a joint tenancy property, then on the demise of your wife, her 20% share in the property devolves to the survivor of joint tenancy, i.e., you. In this situation, your children will not have any rights over your wife's 20% share.
2. However, if it were to be " Tenants in common", then on the demise of your wife, her 20% share in the property devolves equally to her legal heirs, i.e., you and your children.
3. To know exactly as to under which of these clauses the property fits in, we have to look into the recitals in the deed, when both you and your wife bought the property. However, if there is nothing in the recitals regarding joint tenancy or tenents in common, then the tenets in common prevails.
4. Assuming that "Tenants in common" is the indication to be followed, then you have to obtain competent Court's permission to sell minors' share in the property, the necessity for selling, and how the proceeds will be used for welfare of the child/children, their education, etc..
5. Documents required are property documents, birth certificates of your 2 children, background of their current education, your wife's death certificate, etc.