The trail 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-Trail?share
there is a trail head out of the lot - This area has loose rock trail bed. and some elevation change,
so it "might" be ok.
Instead - head up the road and use the trail 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 (?) trail - if you miss it, it will come out on the road at the other parking lot.
Nassau Trail is a 955 m popular blue singletrack trail located near High Bridge New Jersey. This mountain bike primary trail can be used both directions and has a moderate...
www.trailforks.com
View attachment 138841
Take a couple of pics, and report back! Should be an epic day.