The easiest tree run I know of at Breck is Ore Bucket over on Peak 7. I think it's expert rating is only because they want to keep those who don't belong there off it, as the rest of Peak 7 off the Prospector Lift is rolling intermediate terrain.
All the steeper sections of the trail can be avoided easily. There are some very large stumps in there so pay attention. It might be a bit much for your 7 year old if there is more than a couple of inches of powder in there. There are areas there where he will get stuck. I'd ski it once before w/o your son to check it out first.
Though not a trail, the trees to the skiers right of Bonanza can be skied though a little tight (but not for a 7 year old). Ski down Bonanza to where the slow skiing signs are and look for a narrow trail to the right. Drop in anywhere to your left at that point, but keep in mind you will need to exit skiers right from the trees. Try this area before Ore Bucket.