At age 11, she would fit the DR Mini 18 that you have listed here (we sell this chassis).
The short answer is that seat sizing is something we can help you with, the seat charts can at times be confusing, Michael’s suggestions are great!
The long answer is:
One thing I do personally when sizing seats for a driver is first start with their waist (pants) size and their height. Tillett seats is one of the only vendors I know of for seats that has a ‘waist size to seat size’ chart. This allows me to then fit a driver to a Tillet seat, and then I cross this over to whatever other size seat I am comparing to or putting into a kart. Typically a Tillett size L is the same as a size 3 for many other lines of seats, or a C5 or C7 for the NEK, etc. Some of this comes with prior experience and measuring various drivers.
For the DR Mini 18 specifically, the stock standard seat that comes with the kart is a DR Resin GOLD seat, which is made by IMAF. At age 11, if she is a fairly typical 11 year old in terms of size, likely she would fit in a child’s XS, or an adult size 1. Sometimes these seats won’t work, and we will special order an NEK seat in the “A” size range, or we can even go with a special line of Tillett seats called ‘cut-down’ seats that fit drivers with different torso and upper back heights.
Other items you would need to get this kart in particular up and ready for the track would include first and foremost an engine, a proper set of tires for your region, likely some lead ballast / weights, and some other driver comfort accessories depending on the driver.
Hope this helps!