Inexpensive way to determine mass of tiny seeds?
A pharmacy is a public business, and they should have a scale sensitive enough to weigh seeds.  They will have to let you in back to use it of course.  If you show up with her and she has the seeds and she asks the manager nicely, they might let you back and help you use it.  I would show up rather than call; if you show up you will see the manager / pharmacist but if you call you will not talk to the manager.  

If this works she needs to be sure to credit the pharmacy in her project!  

I recommend she weigh several seeds of each type - they might vary.

Your 100 seed idea is a better one, I think.  I understand your sentiment.  



You did not ask about volume but this could be done with a graduated cylinder.  Put some water in it up to, say, 5 ml.  Then drop in a seed.  How much farther does the water level rise?  That is the volume of the seed.