That's essentially what we did at Hunterdon. We just showed up on a Saturday race morning. We both practiced and my daughter raced. They gave us a "one day license" for free to try it out. I think racing was $10 and practice was $5. Rates could vary for Howell as they have lights and such. Long pants and sleeves required, or shorts and short sleeves ok with knee/elbow pads. Helmet is required of course, and full face is a good idea, but I don't think full face is required. We both have full face helmets anyhow.
Unfortunately we joined up right before soccer season started so we haven't made a race since, but we did two Thursday evening practices and she did a beginner clinic which was cool. I bought us each licenses at $60/per the second time we went which are good for 1 year from date purchased. The season is long as it starts April/May weather depending so well worth it. Hell, kind of seems worth it even if we only go a few times.
The track is so smooth, the jumps are really nice, lots of good practice pumping. To me it seems like good MTB practice. I've never been to a mountain bike park, so I don't get a chance to jump "real" jumps often so it's worth it to me just for that. Granted jumping isn't the "fast" way around the track, but either is a steel fatbike.
If you take your kid I def recommend riding with him.