Would this be a fair criticism: "An increase in the federal deficit would be equivalent of mortgaging your grandchildren's future without their...

For the most part, this is a valid criticism.  However, it
is not necessarily completely true.

This statement would
not be true if the increases in the deficit were caused by what you might call
"investments."  If the federal government is spending in ways that will make the economy
and the nation better off down the road, then this is not a problem.  It would be like
investing in your grandchildren's future without their
consent.

However, if the money is being spent in wasteful
ways, ways that do not improve the future chances of the US and its economy, then the
criticism is valid.  In such a case, the government would be taking out loans that our
"grandchildren" would have to pay simply to spend on frivolous
things.

So, the accuracy of this statement depends largely
on the purpose for which the government is borrowing the money.