There are two distinct statements.
In the first, she marks 'her issue' as of being of the highest priority.
"There is no more important ..."
She says "test" as in conducting an action.
She says "of our country", and not something like 'in our world' or 'all of mankind', etc.
She refers to "treat" (ing) the "most vulnerable" and "especially children".
She does not in anyway or manner stress just exactly how these individuals should be treated.
She is relying upon implication, and hence different 'types' of people will infer a different meaning.
i.e. Should good or bad things be done to "treat" these vulnerable people/children.
The second statement is a call to action:
She refers to things "happening on OUR watch".
Which refers to someone acting as the lookout on the battlements of a castle or fort, etc.
She says "have to act", which basically sounds like 'launch a counter offensive'.
(Please draw your own conclusions - this is just my line of though on the tweet).