Dusty the Whale
Mr.Chainsaw
I think most of the trails other than Tiger Woods and the one along the river would be acceptable. There may be something to lift the bike over from time to time. The majority of the trails and within reach for a child.
This is my daughter a few months short of eight years old:
http://www.youtube.com/watch?v=NXCU4JMoPUo
great video. i like the part when she comments about the sand.