Circuit 8 is good, but a bit of a ride for a real little guy to get to.
I saw that Delta Watershed is back open. Bit of a drive, but great for little ones starting out. Not certain if it's still there, but it had a slightly downhill little jump trail. If not out by elementary school is pretty flat.
Another option is Hyannis out to and around where it splits from trail trail down to the river (Bridle, I think). Used to be a 3 foot wide board walk really low to the ground. I had my kids start there, one still had training wheels on. Talked about it for ever about riding on a mtb trail.