Why do you think you need to build up to create a bed?
If you feel it is necessary to add soil for whatever reason, you can use thouroughly composted compost (does that make sense?), stuff that is fairly light and fluffy. That, you can put over tree roots to a depth of 2-3 inches or so, but I wouldn't recommend much more than that. The less the better!
At that point, plant as small of plants as you can find. Plant perennials that are grown in quarts; shrubs use 1 and 2 gallons instead of 3's and 5's. It may take a little longer for stuff to fill out to the size you wanted it to, but the mature trees will thank you.
In basic terms, what happens when you add soil over the root zone of a mature tree (I'm using mature in a loose fashion here), you are reducing the soil aeration around the feeder roots. Without oxygen in the soil, the roots will slowly die. This is the reason for loose, fluffy compost. It will still allow a fair amount of oxygen down to the original root zone.
The reason the compost needs to be thouroughly composted is that if it is still decomposing, it will rob nitrogen out of the soil to finish decomposition. This is NOT good for the tree.
Not knowing exactly what you have in mind, add 1-3 inches of compost, plant as small of plants as you can find, and mulch it. You won't be very well be able to tell the difference between 3 and 5 inches of soil in 2-3 years....
Sorry for the rambling, but hope it helped!
Dan
