ALL financial assets are claims on economic activity
True, but not all claims on economic activity are financial assets. Since, per my example in the end, your kids are going to pay the exact same amount regardless of whether we follow the pretense of the fictitious so called "trust fund" or we don't. So again, where is the economic value in a transaction that changes nothing other than the flow of paper?
Government securities are no different. Government has the power of taxation on the economic production of the nation. That is why they are (usually) the highest rated issuer in the country.
pure deflection, irrelevant to the point. That government has the power to tax doesn't make a loan to itself an asset