OP, brusies are completely normal.
Yes, bruises can be a symptom of something more serious but not all bruises are equal. Once they are yellow/green, they are old bruises and healing.
LotsOfSmallThings has very kindly shown you examples of her daughter's leukemia related bruising and you can see that your daughter's bruises look nothing like that. The shape, location and colour of some of her bruising was, as she said, very obviously not caused by bumping into something.
That is also different to a large bruise developing on an adult's thigh without obvious cause.
Bruises on skin in bony areas, such as knees and shins) is, as others have said, normal and comes from just bumping into things, falling over etc and don't even need much impact in children. Some people do bruise more easily than others.
I used to be covered in bruises after a night dancing in a mosh pit which I always equate with childen running around in a playground!
I was also 'covered in bruises' (including soft areas) as a child but that was a result of being hit at home.
If you are really worried, speak to your GP for reassurance (after all, opinions on a MN thread does not equal a medical diagnosis) but bruises on knees or shins, no where else (particularly soft areas), with no other symptoms and, tbh, I wouldn't even have wondered if I should be worried when mine were children.
But also, bruises on soft areas do happen I had a cracking bruise on my thigh a few months ago after walking into the corner of a table in the classroom! Sometimes, I notice a bruise and have no recollection of walking into a table but assume I must have done.
Bruising like your daughter's wouldn't concern me.