So at least 0,9% have real food problems, thats we can all agree on, right?
That means in the US, with the highest per-capita income of all big states, nearly 3 million people hunger or are malnourished.
I think all agree that, for various reasons, there are children not receiving proper nutrition - and, pretty much by definition (i.e., because they are children) this is through no fault of their own.
There are also adults not receiving proper nutrition. Here there is a split - for some (e.g., genetic medical problems) it is also through no fault of their own, for others it is due to poor life choices.
The 0.9% is out of "U.S. households with children" so the denominator is 38.5 million. Of course there could be >1 child per household. Assuming 2 children/household, we get 0.9% * 38.5 million * 2 = ~700,000.
That is still a very large number, but nowhere near the size implied by the misleading "one in five American kids..." quote. Whenever people - from any political perspective - cite a misleading statistic, it causes me to think less of their opinion. YMMV.
As for "how much underfed?" - that is difficult to know. One could posit that people would not admit to a problem unless it is severe (leading to an under count of problems), or posit that counting "ever in the past 12 months" is not indicative of a real problem (leading to an over count of real problems).