Sorry to be the bearer of bad news...but it is a VERY bad idea to fill that in. When you look at a trunk of a tree, you should see a flare at the base where it enters the ground. If it looks like a post stuck in the ground, it us unhealthy/fatal to the tree. it might take 15-20 years...but if it is buried too deep (especially Norway maple...), it has a rather low probability of making it to 40.
There are a couple of problems:
1) when trunk tissue is wet all of the time it can start to rot. It us supposed to dry out. This happens, but not all the time.
2) What happens more frequently is that roots grow up into that soil/mulch/compost/whatever is piled there. Those roots grow tangential to the tree's trunk or sometimes around the trunk. As the trunk gets bigger, it grows into those roots and chokes itself. It is a problem called "stem girdling roots".
I know you just spent time/money doing the bricks, but my recommendation would be to remove those. If you want those as a border, do just one course - maybe even set that into the ground an inch or two. Fill that area with mulch. Pull the mulch away from the trunk of the trees. Then plant whatever you want in that mulch bed.
(As an aside, many trees are coming deep from the nurseries...then get planted deeper by contractors who don't know or care about long-term tree health. I wouldn't be surprised if the tree is already deep...hopefully now...but unfortunately that is a reality we deal with all the time. Post a picture of two of where the trunk enters the ground).