Hi Ania!
We used Singapore for homeschooling our 6 year old after pulling him out of first grade math. We did end up using the second book for first grade, to get a good review and finish up first grade stuff, as Singapore included some work that wasn't done in the Saxon program they use at his school.
But I found that when we moved into the 2nd grade book there was still a lot of repetition and so we skipped a bit here and there and used supplemental workbooks and "challenge math" stuff. I have heard from others who use Singapore that the basic texts/workbooks are good but that to really challenge you should use the "Challenge Word Problems" books. To me, it seems that Singapore is really good with hitting the basics hard. But if she's already got that down, then maybe she just needs more challenge! I say go for the 6th grade stuff and just supplement with other stuff (Singapore or not) as needed and/or interested. I'd say follow her lead and don't worry too much about the grade level printed on the cover of the book.
