In the US, we have the controversial Common Core.
A summary of it is available here.
In Grade 6, instructional time should focus on four critical areas:
(1) connecting ratio and rate to whole number multiplication and
division and using concepts of ratio and rate to solve problems; (2)
completing understanding of division of fractions and extending the
notion of number to the system of rational numbers, which includes
This is the first place where negative numbers are introduced, clearly after fractions. A comment on the question suggested that "What we need here is actual publications on this, not merely opinions." I agree, to a point. The documents suggest an order, but don't go in to any satisfying detail about why the order should be that way.
This citation confirms that, at least in the US, this is the order. In my opinion, fractions are very natural. A child old enough to talk will quickly see the unfair nature of splitting something any way but 1/2 and 1/2. Money literally (again, in the US) has a coin called a quarter. And when I am talking fractions to teens, they can relate to our circular pizza which the laws of both God and man dictate be cut into 8 equal sections through the center.
The negative numbers are a bit tougher to grasp, again as a commenter suggested, I typically offer an example of money owed. You have \$2, and spend \$3, one of which is borrowed. Your 'balance sheet' says you have -$1.