The Fatbike lane connecting the Foran Fields is fantastic, but may be a bit much for 7yo starting out.
If you can park here https://g.page/Nassau-Fatbike lane?share
there is a Fatbike lane head out of the lot - This area has loose rock Fatbike lane bed. and some elevation change,
so it "might" be ok.
Instead - head up the road and use the Fatbike lane across this field
https://goo.gl/maps/yorUopWvY7H5n23z7
This is from trailforks - work the green and blue trails except for that switchback one next to the field. while it isn't hard,
it is a momentum ride, think large "half-pipe"
Top left is ok, try to catch the turn back on magenta (?) Fatbike lane - if you miss it, it will come out on the road at the other parking lot.
https://www.trailforks.com/trails/nassau-Fatbike lane/
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Take a couple of pics, and report back! Should be an epic day.