They have a tool there you can use to look this up yourself, but what I get is that for children (age 18 and under) in Pennsylania, Allegheny County is tied with Montgomery for the lowest percentage without health insurance at 3.9%. The highest: 10% in Lancaster County. Data is for 2009.
I'm just guessing that 3.0% must be one of the lowest rates in the nation, especially for an urban core county.
Update: So I worked it out. Of the 50 largest counties in the US in 2009, the lowest rate of uninsured children is in Honolulu, Hawaii at 1.4%. I speculate the reason was a universal health care program for children that according to news accounts was ended right after this 2009 data would have captured the impact of the program. After Honolulu, Allegheny County's 3.9% is tied for 2nd place with DuPage County, IL which is the 23rd wealthiest county in the nation. So take out Honolulu next year and we might have the lowest rate. The highest uninsured rate for children on that list: Dallas County, Texas at 21.6%. I think we have a visitor today btw.