In order to accurately find out your chilren’s blood quantum, you would need to do their basic genealogy, tracing them to their most recently tribally-recognized ancestor. You MUST be able to locate this person and get actual verification from the tribe that the person was/is recognized to even begin to get started. Using that person’s blood quantum you’d calculate your children’s blood quantum, dividing the blood quantum in half for each generation following that person.

For example, using one of my own grandparents:
*My grandmother is enrolled in the Rocky Boy Chippewa-Cree Tribe (This is my "most recently recognized ancestor" from that tribe.)
*She is recognized as being 5/8 Chippewa-Cree (No distinction is made between how much Chippewa and how much Cree on her tribal enrollment paperwork, which I have a copy of directly from the Rocky Boy Chippewa-Cree Tribe)
*My dad gets half of his mother’s Chippewa-Cree blood making my dad 5/16 Chippewa-Cree. (My dad, however, is not enrolled in Rocky Boy although he is eligible for enrollment there. Instead he’s enrolled in his dad’s, my grandfather’s, tribe.)
*I get half of my dad’s Chippewa-Cree blood making me 5/32 Chippewa-Cree. (Like my dad though I am not enrolled in Rocky Boy nor would I be eligible for enrollment there as I do not meet their enrollment requirements. However, I do meet the enrollment requirements for another tribe, that of both my father and mother, and I am enrolled there instead.)
*My kids get half of my Chippewa-Cree blood making them 5/64 but at this point, although it exists in their being and they love my grandma (their great grandma) dearly, it isn’t a part of their identities as they’re being raised among the people of my and their father’s tribes. It doesn’t mean they don’t have Chippewa-Cree ancestry (obviously they do) it just means they aren’t recognized as being Chippewa-Cree by the Rocky Boy Tribe.

There is no "guess work" involved, only research and math. And certainly not any blood test.

I have done a lot of research on DNA testing and my findings are on my website @
http://www.coolrogue.net/genealogy/dnatestingpt1.html

It gives you a percentage breakdown of how your DNA seems to be composed of European, Sub-Saharan African, East Asian and Native American.

When you see your results however, you do have to consider what you know of your family history before you interpret the numbers. Some areas of Europe have genetic material that is Central Asian, and that can come through as Native American in the results. So you have to do a little research before interpreting the results at face value. "Europe" in the genetic sense also includes North Africa and Mesopotamia, to Northern India (the genetic Indo-European areas).

So seeing a result like 15% Native American could mean that 5% may be Central Asian from when the Huns invaded your European ancestors, and that 10% is really due to the New World ancestors.

Also, it can’t tell you if you are of a specific tribe. I think First Nations groups require some sort of traditional genealogy to claim affinity with them.