How do I indicate to give a one-off specific gift to a particular person only if he/she assumes the role of guardian to my minor child(ren)?

Giving a specific gift from your residual assets. Under the "Select Clauses" tab, toggle on "Other Instructions" and key in the following:

I give my {Relationship}, {Name}, {Identification Number/details}, a specific gift of {Currency}{Amount} to be drawn from my Residual Assets as a token of appreciation only if he/she assumes the role of guardian to my minor children.

Giving a specific gift from a specific asset. Under the “Allocate Assets” tab, within that specific asset toggle on " Additional instructions" and key in the following:

I give my {Relationship}, {Name}, {Identification Number/details}, a specific gift of {Currency}{Amount} to be drawn from this asset as a token of appreciation only if he/she assumes the role of guardian to my minor children.