Merwin, MO is just a mile from the KS line, and south of LV by about 60-90 minutes. It has a practice area, a pit bike track, a main outdoor-style MX track, and several miles of trails. Deepwater is a ways further, and I would not recommend going there unless it is a weekday. It is a drunken, hillbilly, idoits-on-parade free-for-all on the weekends. It is 6 miles south of Clinton, MO on hwy 13. Kingsville, MO is about 30 miles east of the state line, between state highways 50 and 58. It is located about half way between those two highways on county highway "W". If you find Kingsville on a map, the track is about 3 miles north of there on W. Kingsville proper is at 58 and W. Kingsville has kids-only practice days on Tuesday, although some of them little boogers are scary fast! They have a supercross-style track, and a limited (unprepped) practice area. Also, up around St. Joseph, MO is Knobby Hill MX. It is a full size, MX only park. Not sure about their schedule, though.
Kingsville and Merwin are both owned by RJ Wohlberg. I think it's $5 for 50cc bikes at each place. Feel free to call RJ if you have any questions about either place. 816.456.7223
Tell him Louis referred you.